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

[Chris B via cfe-commits]

llvm-beanz wrote:

@etcwilde's point above is that the `CLANG_RESOURCE_DIR` is already available in CMake for projects that import the Clang package or are built in-tree with Clang. You can get the directory via CMake with:

```cmake
get_target_property(CLANG_IMPORT_HEADERS clang-resource-headers INTERFACE_INCLUDE_DIRECTORIES)
```

That should make this change unnecessary even in the case where you need the resource directory in CMake.

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