[llvm-branch-commits] Reland "[compiler-rt] Allow running tests without installing first" (PR #88075)

[Alexander Richardson via llvm-branch-commits]

https://github.com/arichardson created https://github.com/llvm/llvm-project/pull/88075

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}`.

The existing logic works fine when clang and compiler-rt share the same
build directory ``-DLLVM_ENABLE_PROJECTS=clang;compiler-rt`, but when
building compiler-rt separately we need to tell the compiler used for
the tests where it can find the just-built libraries.

This reduces the fixes check-all failures to one 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
```

This relands the previous PR with fixes for Windows.
Depends on https://github.com/llvm/llvm-project/pull/88074 to be merged
first for GCC buildbots.