[llvm] RuntimeLibcalls: Add stub API for getting function signatures (PR #166290)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-sie-ubuntu-fast` running on `sie-linux-worker` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/144/builds/39324

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: Transforms/Util/DeclareRuntimeLibcalls/sincos_stret.ll' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/opt -S -passes=declare-runtime-libcalls -mtriple=x86_64-apple-macos10.9 < /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/sincos_stret.ll | /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/FileCheck -check-prefixes=CHECK,X64 /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/sincos_stret.ll
# executed command: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/opt -S -passes=declare-runtime-libcalls -mtriple=x86_64-apple-macos10.9
# note: command had no output on stdout or stderr
# executed command: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/FileCheck -check-prefixes=CHECK,X64 /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/sincos_stret.ll
# note: command had no output on stdout or stderr
# RUN: at line 2
/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/opt -S -passes=declare-runtime-libcalls -mtriple=arm64-apple-macos10.9 < /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/sincos_stret.ll | /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/FileCheck -check-prefixes=CHECK,STRUCT /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/sincos_stret.ll
# executed command: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/opt -S -passes=declare-runtime-libcalls -mtriple=arm64-apple-macos10.9
# .---command stderr------------
# | /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/opt: warning: failed to infer data layout: unable to get target for 'arm64-apple-macos10.9', see --version and --triple.
# | /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/opt: WARNING: failed to create target machine for 'arm64-apple-macos10.9': unable to get target for 'arm64-apple-macos10.9', see --version and --triple.
# `-----------------------------
# executed command: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/FileCheck -check-prefixes=CHECK,STRUCT /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/sincos_stret.ll
# note: command had no output on stdout or stderr
# RUN: at line 3
/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/opt -S -passes=declare-runtime-libcalls -mtriple=thumbv7k-apple-watchos2.0 < /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/sincos_stret.ll | /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/FileCheck -check-prefixes=CHECK,STRUCT /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/sincos_stret.ll
# executed command: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/opt -S -passes=declare-runtime-libcalls -mtriple=thumbv7k-apple-watchos2.0
# .---command stderr------------
# | /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/opt: warning: failed to infer data layout: unable to get target for 'thumbv7k-apple-watchos2.0', see --version and --triple.
# | /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/opt: WARNING: failed to create target machine for 'thumbv7k-apple-watchos2.0': unable to get target for 'thumbv7k-apple-watchos2.0', see --version and --triple.
# `-----------------------------
# executed command: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/FileCheck -check-prefixes=CHECK,STRUCT /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/sincos_stret.ll
# note: command had no output on stdout or stderr
# RUN: at line 4
/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/opt -S -passes=declare-runtime-libcalls -mtriple=armv7-apple-ios7 < /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/sincos_stret.ll | /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/FileCheck -check-prefix=SRET /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/sincos_stret.ll
# executed command: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/opt -S -passes=declare-runtime-libcalls -mtriple=armv7-apple-ios7
# .---command stderr------------
# | /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/opt: warning: failed to infer data layout: unable to get target for 'armv7-apple-ios7', see --version and --triple.
# | /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/opt: WARNING: failed to create target machine for 'armv7-apple-ios7': unable to get target for 'armv7-apple-ios7', see --version and --triple.
# `-----------------------------
# executed command: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/FileCheck -check-prefix=SRET /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/sincos_stret.ll
# .---command stderr------------
# | /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/sincos_stret.ll:17:9: error: SRET: expected string not found in input
# | ; SRET: declare void @__sincos_stret(ptr sret({ double, double }) align 4, double) [[SINCOS_ATTRS:#[0-9]+]]
# |         ^
# | <stdin>:602:76: note: scanning from here
# | declare void @__sincosf_stret(ptr sret({ float, float }) align 4, float) #0
# |                                                                            ^
# | <stdin>:605:1: note: possible intended match here
# | declare void @__sincos_stret(ptr sret({ double, double }) align 8, double) #0
# | ^
# | 
# | Input file: <stdin>
# | Check file: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/sincos_stret.ll
...

```

</details>

https://github.com/llvm/llvm-project/pull/166290