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

[Kim Gräsman via cfe-commits]

kimgr wrote:

> The LLVM_EXTERNAL_PROJECT mode solution using $<TARGET_FILE:clang> won't work so well,
> as it pins the resource directory to the build tree. So it won't resolve correctly after install, I suppose.

Ah, in that mode I guess we could use the default behavior, and not override the resource dir.

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