[PATCH] D32576: [Modules] Improve diagnostics for incomplete umbrella

[Duncan P. N. Exon Smith via Phabricator via cfe-commits]

dexonsmith added a comment.

Oh, and it would be nice to split out `Preprocessor::diagnoseMissingHeaderInUmbrellaDir` in a separate NFC commit ahead of time.


https://reviews.llvm.org/D32576