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

[Chris B via cfe-commits]

llvm-beanz wrote:

(cc @compnerd, @gottesmm, & @etcwilde who have played roles in building and maintaining all the build-tree stuff for Swift).

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