[clang] [clang][modules] Read PCM validation timestamp earlier (PR #177062)

[Cyndy Ishida via cfe-commits]

cyndyishida wrote:

>this fix is rather urgent, so I think not breaking any existing tests is sufficient.

I don't agree. The last resolution for classes of this problem also landed without a test, and we saw it reappear. I'm not saying we need a regression test here to prove correctness, but there needs to be some way to provide confidence that this fixes the bug observed.

https://github.com/llvm/llvm-project/pull/177062