[clang] [Modules][Diagnostic] Mention which AST file's options differ from the current TU options. (PR #101413)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

> Sorry that the change is pretty big. But it is fairly mechanical. If you have ideas about simplifying the review process, I'm open to that. Right now I'm thinking if I should make the logical changes first, you review them, and I commit clang-formatted change.

I'd suggest keeping this PR and the resulting commit free of any formatting changes and then commit the results from `clang-format` as a separate commit.

After https://github.com/llvm/llvm-project/pull/101280 I had the impression that we're trying to be very precise about reporting whether an AST file is a precompiled header, module file, or other. Why doesn't this PR implement that? To be clear this change is already a big improvement as is, but I'd like to understand if further polish and unification is still planned.

https://github.com/llvm/llvm-project/pull/101413