[clang] [clang] NFC: new tests and some cleanups on existing ones (PR #142293)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-sve-vls-2stage` running on `linaro-g3-04` while building `clang` at step 12 "ninja check 2".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/4/builds/7116

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

```
Step 12 (ninja check 2) failure: stage 2 checked (failure)
...
clang++: warning: argument unused during compilation: '-mllvm -treat-scalable-fixed-error-as-warning=false' [-Wunused-command-line-argument]
[1196/1388] Linking CXX executable unittests/Target/TargetMachineCTests
clang++: warning: argument unused during compilation: '-mllvm -treat-scalable-fixed-error-as-warning=false' [-Wunused-command-line-argument]
[1205/1388] Linking CXX executable unittests/Target/PowerPC/PowerPCTests
clang++: warning: argument unused during compilation: '-mllvm -treat-scalable-fixed-error-as-warning=false' [-Wunused-command-line-argument]
[1212/1388] Linking CXX executable unittests/TableGen/TableGenTests
clang++: warning: argument unused during compilation: '-mllvm -treat-scalable-fixed-error-as-warning=false' [-Wunused-command-line-argument]
[1213/1388] Linking CXX executable unittests/Telemetry/TelemetryTests
clang++: warning: argument unused during compilation: '-mllvm -treat-scalable-fixed-error-as-warning=false' [-Wunused-command-line-argument]
[1222/1388] Linking CXX executable unittests/Testing/Support/TestingSupportTests
FAILED: unittests/Testing/Support/TestingSupportTests 
: && /home/tcwg-buildbot/worker/clang-aarch64-sve-vls-2stage/stage1.install/bin/clang++ -mcpu=neoverse-512tvb -msve-vector-bits=256 -mllvm -treat-scalable-fixed-error-as-warning=false -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wc++98-compat-extra-semi -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wno-comment -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -fuse-ld=lld -Wl,--color-diagnostics    -Wl,--gc-sections unittests/Testing/Support/CMakeFiles/TestingSupportTests.dir/TempPathTest.cpp.o -o unittests/Testing/Support/TestingSupportTests  lib/libLLVMSupport.a  lib/libLLVMTestingSupport.a  lib/libLLVMSupport.a  lib/libllvm_gtest_main.a  lib/libllvm_gtest.a  lib/libLLVMTestingSupport.a  lib/libllvm_gtest.a  lib/libLLVMSupport.a  -lrt  -ldl  -lm  /usr/lib/aarch64-linux-gnu/libz.so  lib/libLLVMDemangle.a  -lpthread && :
clang++: warning: argument unused during compilation: '-mllvm -treat-scalable-fixed-error-as-warning=false' [-Wunused-command-line-argument]
ld.lld: error: cannot open lib/libllvm_gtest.a: No such file or directory
ld.lld: error: cannot open lib/libllvm_gtest.a: No such file or directory
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
[1284/1388] Building CXX object unittests/Frontend/CMakeFiles/LLVMFrontendTests.dir/OpenMPDecompositionTest.cpp.o
[1286/1388] Building CXX object unittests/Transforms/Instrumentation/CMakeFiles/InstrumentationTests.dir/MemProfUseTest.cpp.o
[1287/1388] Building CXX object unittests/Transforms/Scalar/CMakeFiles/ScalarTests.dir/LoopPassManagerTest.cpp.o
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/142293