[cmake] Expose clang resource dir to installed CMake

[kimgr]

In IWYU's build, we guess Clang's resource dir based on the CMAKE_PREFIX_PATH, so we can collect the builtin/resource headers for our own build tree. Unfotunately, depending on packaging/platform, the path to the resource dir varies quite a bit.

Is there any chance to expose something like CLANG_RESOURCE_DIR in ClangConfig.cmake?

It's not fully clear to me what the layering looks like there, but it feels like it should be possible.

Thanks!

[kimgr]

(I'd be happy to start on a patch, but I need a thread to start pulling on. Is there an example patch doing something similar to give me an idea what would need to change? Thanks!)

[kimgr]

Eventually landed #103388 to support this. I'll ping back here once I have everything wired in IWYU and can explain how to do it properly.