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

[Sam McCall via cfe-commits]

sam-mccall wrote:

Yes, it's approximately the same problem. Thanks & sorry for the noise!

(We have a non-clang include-scanner that computes dependencies to ensure hermetic builds. The indirect include defeats the include scanner, so we were accidentally relying on `<vector>` being available for some other reason - which I suppose was this one).

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