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

[Kim Gräsman via cfe-commits]

kimgr wrote:

@tstellar @tuliom It seems to me _almost_ all callers pass `CLANG_RESOURCE_DIR` for the `CustomResourceDir` optional argument:
https://github.com/search?q=repo%3Allvm%2Fllvm-project%20Driver%3A%3AGetResourcesPath&type=code

the one exception being:
https://github.com/llvm/llvm-project/blob/d72c8b02802c87386f5db3c7de6c79e921618fa3/clang/tools/libclang/CIndexer.cpp#L156

I wonder if that last one is a bug...? If so, it seems it would be better to just fold `CLANG_RESOURCE_DIR` into `Driver::GetResourcesPath` and remove the optional argument.

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