[clang] Ensure `--print-runtime-dir` path exists (#102834)
Before this PR, `clang --print-runtime-dir` on Windows used to report a
non-existent directory if `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=OFF`.
We now check if any of the known runtime directories exist before
printing any of them on stdout. If none exists, we print `(runtime dir
is not present)`.
