[flang-commits] [flang] [flang] Support discovering LLVM/Clang/MLIR without explicit *_DIR (PR #122639)

[Leandro Lupori via flang-commits]

https://github.com/luporl approved this pull request.

LGTM, thanks for preserving the old behavior.

I've tested these changes locally and standalone flang builds work.

https://github.com/llvm/llvm-project/pull/122639