[clang] [clang][modulemap] Lazily load module maps by header name (PR #181916)

[Jan Svoboda via cfe-commits]

https://github.com/jansvoboda11 commented:

This is a large patch, so a have a bunch of questions to make sure I'm understanding this right. Some of those explanations might make sense to put directly into code comments. I love the remarks-based tests!

https://github.com/llvm/llvm-project/pull/181916