[clang] [lanai] Add back the ability to set the memory model with --mcmodel. (PR #150575)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-quick` running on `linaro-clang-aarch64-quick` while building `clang` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/65/builds/21486

<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 'Clangd Unit Tests :: ./ClangdTests/63/331' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests-Clangd Unit Tests-2694648-63-331.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=331 GTEST_SHARD_INDEX=63 /home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests
--

Note: This is test shard 64 of 331.
[==========] Running 4 tests from 4 test suites.
[----------] Global test environment set-up.
[----------] 1 test from ClangdTests
[ RUN      ] ClangdTests.PreambleVFSStatCache
ASTWorker building file /clangd-test/foo.cpp version null with command 
[/clangd-test]
clang -ffreestanding /clangd-test/foo.cpp
Driver produced command: cc1 -cc1 -triple aarch64-unknown-linux-gnu -fsyntax-only -disable-free -clear-ast-before-backend -main-file-name foo.cpp -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=non-leaf -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -ffreestanding -enable-tlsdesc -target-cpu generic -target-feature +v8a -target-feature +fp-armv8 -target-feature +neon -target-abi aapcs -debugger-tuning=gdb -fdebug-compilation-dir=/clangd-test -fcoverage-compilation-dir=/clangd-test -resource-dir lib/clang/22 -internal-isystem lib/clang/22/include -internal-isystem /usr/local/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -fdeprecated-macro -ferror-limit 19 -fno-signed-char -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fcxx-exceptions -fexceptions -no-round-trip-args -target-feature -fmv -faddrsig -D__GCC_HAVE_DWARF2_CFI_ASM=1 -x c++ /clangd-test/foo.cpp
Building first preamble for /clangd-test/foo.cpp version null
not idle after addDocument
UNREACHABLE executed at ../llvm/clang-tools-extra/clangd/unittests/SyncAPI.cpp:22!
 #0 0x0000c113b5ddc584 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests+0xc5c584)
 #1 0x0000c113b5dda04c llvm::sys::RunSignalHandlers() (/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests+0xc5a04c)
 #2 0x0000c113b5ddd3e0 SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
 #3 0x0000e3e3c93538f8 (linux-vdso.so.1+0x8f8)
 #4 0x0000e3e3c8ebf1f0 __pthread_kill_implementation ./nptl/./nptl/pthread_kill.c:44:76
 #5 0x0000e3e3c8e7a67c gsignal ./signal/../sysdeps/posix/raise.c:27:6
 #6 0x0000e3e3c8e67130 abort ./stdlib/./stdlib/abort.c:81:7
 #7 0x0000c113b5d88e80 llvm::RTTIRoot::anchor() (/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests+0xc08e80)
 #8 0x0000c113b5c33d68 clang::clangd::runCodeComplete(clang::clangd::ClangdServer&, llvm::StringRef, clang::clangd::Position, clang::clangd::CodeCompleteOptions) (/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests+0xab3d68)
 #9 0x0000c113b5762fa0 clang::clangd::(anonymous namespace)::ClangdTests_PreambleVFSStatCache_Test::TestBody() ClangdTests.cpp:0:0
#10 0x0000c113b5e33de0 testing::Test::Run() (/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests+0xcb3de0)
#11 0x0000c113b5e35104 testing::TestInfo::Run() (/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests+0xcb5104)
#12 0x0000c113b5e35d40 testing::TestSuite::Run() (/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests+0xcb5d40)
#13 0x0000c113b5e460c0 testing::internal::UnitTestImpl::RunAllTests() (/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests+0xcc60c0)
#14 0x0000c113b5e45a0c testing::UnitTest::Run() (/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests+0xcc5a0c)
#15 0x0000c113b5e20b28 main (/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests+0xca0b28)
#16 0x0000e3e3c8e673fc __libc_start_call_main ./csu/../sysdeps/nptl/libc_start_call_main.h:74:3
#17 0x0000e3e3c8e674cc call_init ./csu/../csu/libc-start.c:128:20
#18 0x0000e3e3c8e674cc __libc_start_main ./csu/../csu/libc-start.c:379:5
#19 0x0000c113b55df7f0 _start (/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests+0x45f7f0)

--
exit: -6
--
shard JSON output does not exist: /home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests-Clangd Unit Tests-2694648-63-331.json
********************


```

</details>

https://github.com/llvm/llvm-project/pull/150575