[llvm] [SandboxIR] Implement VAArgInst (PR #106247)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-armv8-quick` running on `linaro-clang-armv8-quick` while building `llvm` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/154/builds/3403

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

```
Step 5 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'ClangPseudo :: cxx/dangling-else.cpp' FAILED ********************
Exit Code: 2

Command Output (stderr):
--
RUN: at line 1: clang-pseudo -grammar=cxx -source=/home/tcwg-buildbot/worker/clang-armv8-quick/llvm/clang-tools-extra/pseudo/test/cxx/dangling-else.cpp --start-symbol=statement-seq --print-forest | /home/tcwg-buildbot/worker/clang-armv8-quick/stage1/bin/FileCheck /home/tcwg-buildbot/worker/clang-armv8-quick/llvm/clang-tools-extra/pseudo/test/cxx/dangling-else.cpp
+ /home/tcwg-buildbot/worker/clang-armv8-quick/stage1/bin/FileCheck /home/tcwg-buildbot/worker/clang-armv8-quick/llvm/clang-tools-extra/pseudo/test/cxx/dangling-else.cpp
+ clang-pseudo -grammar=cxx -source=/home/tcwg-buildbot/worker/clang-armv8-quick/llvm/clang-tools-extra/pseudo/test/cxx/dangling-else.cpp --start-symbol=statement-seq --print-forest
clang-pseudo: ../llvm/clang-tools-extra/pseudo/lib/cxx/CXX.cpp:437: auto clang::pseudo::cxx::getLanguage()::(anonymous class)::operator()() const: Assertion `Diags.empty()' failed.
#0 0x0076ed24 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/tcwg-buildbot/worker/clang-armv8-quick/stage1/bin/clang-pseudo+0x5ed24)
#1 0x0076cab4 llvm::sys::RunSignalHandlers() (/home/tcwg-buildbot/worker/clang-armv8-quick/stage1/bin/clang-pseudo+0x5cab4)
#2 0x0076f778 SignalHandler(int) Signals.cpp:0:0
#3 0xf7a8d6e0 __default_sa_restorer ./signal/../sysdeps/unix/sysv/linux/arm/sigrestorer.S:67:0
#4 0xf7a7db06 ./csu/../sysdeps/unix/sysv/linux/arm/libc-do-syscall.S:47:0
#5 0xf7abd292 __pthread_kill_implementation ./nptl/pthread_kill.c:44:76
#6 0xf7a8c840 gsignal ./signal/../sysdeps/posix/raise.c:27:6
FileCheck error: '<stdin>' is empty.
FileCheck command line:  /home/tcwg-buildbot/worker/clang-armv8-quick/stage1/bin/FileCheck /home/tcwg-buildbot/worker/clang-armv8-quick/llvm/clang-tools-extra/pseudo/test/cxx/dangling-else.cpp

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/106247