[llvm-branch-commits] [compiler-rt] Allow running tests without installing first (PR #83088)

[Alexander Richardson via llvm-branch-commits]

https://github.com/arichardson created https://github.com/llvm/llvm-project/pull/83088

Currently, the testsuite uses the default runtimes path to find the
runtimes libraries which may or may not match the just-built runtimes.
This change uses the -resource-dir flag for clang whenever
COMPILER_RT_TEST_STANDALONE_BUILD_LIBS is set to ensure that we are
actually testing the currently built libraries rather than the ones
bundled with ${COMPILER_RT_TEST_COMPILER}.

This mostly fixes check-all in my configuration:
```
cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo -G Ninja
-DCMAKE_C_COMPILER=$HOME/output/upstream-llvm/bin/clang
-DCMAKE_CXX_COMPILER=$HOME/output/upstream-llvm/bin/clang++
-DCOMPILER_RT_INCLUDE_TESTS=ON
-DLLVM_EXTERNAL_LIT=$HOME/build/upstream-llvm-project-build/bin/llvm-lit
-DLLVM_CMAKE_DIR=$HOME/output/upstream-llvm
-DCOMPILER_RT_DEBUG=OFF
-S $HOME/src/upstream-llvm-project/compiler-rt
-B $HOME/src/upstream-llvm-project/compiler-rt/cmake-build-all-sanitizers
```

I am still seeing two test failures due to an msan error inside
__gxx_personality_v0, but that is most likely due to an uninstrumented
`/lib/x86_64-linux-gnu/libgcc_s.so.1` being used by these tests.