[clang] [CMake] Deprecate GCC_INSTALL_PREFIX (PR #77537)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> Seems okay to me, but it would be good to include an update to the release notes, which ideally shows an example of what replacing the cmake flag with a config file would look like.

Thanks for the feedback. I have added pseudocode how GCC installation detection works in the description and changed release note/CMake warning to mention this pull request.

https://github.com/llvm/llvm-project/compare/36acf7d2bdcdcfae5a60259a9a6f4869419d721e..8bd31c3cdb6ce41194b063c9ac13e5e2fbfbcc02

https://github.com/llvm/llvm-project/pull/77537