[clang] RFC: [cmake] Export CLANG_RESOURCE_DIR in ClangConfig (PR #97197)

[Tulio Magno Quites Machado Filho via cfe-commits]

tuliom wrote:

> I wonder if that last one is a bug...?

Agreed.

> If so, it seems it would be better to just fold `CLANG_RESOURCE_DIR` into `Driver::GetResourcesPath` and remove the optional argument.

Or making `CustomResourceDir = CLANG_RESOURCE_DIR` by default instead of `""`.
This would allow users to continue setting a different resource directory than `CLANG_RESOURCE_DIR`, although I still don't understand in which case that would be used.

https://github.com/llvm/llvm-project/pull/97197