[runtimes] Pass-through CLANG_RESOURCE_DIR to cmake invocations

[tstellar]

compiler-rt and libomp need access to this variable in order to install their libraries and headers to the path that clang expects.

[kwk]

This looks good to me and for the day that I'm testing it brings down the number of failing tests from 53 to 23.

[petrhosek]

What's issue you're seeing without this change? Projects like compiler-rt and libomp use https://github.com/llvm/llvm-project/blob/a8837b49c1c1650357d7d8f2957c903b3a5ab039/cmake/Modules/GetClangResourceDir.cmake to retrieve the Clang resource directory which will construct it manually when CLANG_RESOURCE_DIR isn't set and I haven't seen any issue being reported.

[tstellar]

@petrhosek This issue we've seen is some test case failures caused by clang and compiler-rt disagreeing on where the clang resource directory is.

If you do something like cmake -DCLANG_RESOURCE_DIR=../lib/clang/18, clang will use this path, but when compiler-rt calls get_clang_resource_dir, CLANG_RESOURCE_DIR is not set (because it's a different CMake invocation), so it tries to construct it's own path and ends up with something different.

[petrhosek]

That makes sense. I'd prefer having a list of passthrough variables and then include CLANG_RESOURCE_DIR in that list as a more generic mechanism rather than using COMMON_CMAKE_ARGS but I'm also fine landing this change as is and refactoring it later.