[llvm] [runtimes] Pass-through CLANG_RESOURCE_DIR to cmake invocations (PR #73185)

[Tom Stellard via llvm-commits]

tstellar wrote:

@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.

https://github.com/llvm/llvm-project/pull/73185