[clang] [llvm] [Flang] Search flang_rt in clang_rt path (PR #151954)

[Michael Kruse via cfe-commits]

Meinersbur wrote:

For reference. compiler-rt special Apple code:

https://github.com/llvm/llvm-project/blob/9b2b5bf8cd751fabccaca1b8847b4c291aa52081/compiler-rt/cmake/Modules/CompilerRTUtils.cmake#L518

flang-rt special Apple code:
https://github.com/llvm/llvm-project/blob/9b2b5bf8cd751fabccaca1b8847b4c291aa52081/flang-rt/cmake/modules/GetToolchainDirs.cmake#L37

https://github.com/llvm/llvm-project/pull/151954