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

[Kim Gräsman via cfe-commits]

kimgr wrote:

@llvm-beanz Thanks for the pointers!

Here's what I ended up with as a first step:
https://github.com/include-what-you-use/include-what-you-use/commit/b03f60adb2a9fa884d35bea5eb77c9e8a05d80c0

I couldn't get the `get_target_property` spelling you suggested to work (it doesn't resolve when building against the Debian packages), so I ended up with something else.

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.

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