[clang] [clang][modules] Stop eagerly reading files with diagnostic pragmas (PR #87442)

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alexfh wrote:

> @alexfh Can you check it doesn't boil down to the same thing as the issue you reported in https://reviews.llvm.org/D137213?

Oh, that was long ago ;) And indeed, as @sam-mccall said, it turned out to be the problem of our build setup. Sorry for the noise!

https://github.com/llvm/llvm-project/pull/87442