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

[Petr Hosek via llvm-commits]

petrhosek wrote:

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.

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