[clang] [clang][modules] Headers meant to be included multiple times can be completely invisible in clang module builds (PR #83660)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

I still don't understand why the number of `FileID`s for module maps has increased so much. I am observing this increase when compiling a file that has many module dependencies, it creates only one `FileID` for a module map I mentioned above, the rest 2259 are from loaded binary PCM files.

https://github.com/llvm/llvm-project/pull/83660