[clang][modulemap] Fix crashes loading pcms (#218011)
Previously in the case that the directory was missing, loading a pcm
would crash. Now it attempts to load absolute paths first, and
gracefully ignores missing directories.
Follow up to https://github.com/llvm/llvm-project/pull/181916
Fixes: https://github.com/llvm/llvm-project/issues/215931
Assisted By: codex
