[clang] [clang][OpenMP] Keep 'requires' directives read from an AST file (PR #220058)

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pgerell wrote:

I've read the LLVM contribution and AI tool policies. I've reviewed and understand the changes in this PR and take responsibility for them; the AI assistance is disclosed in the PR description.

@alexey-bataev @ChuanqiXu9, would either of you be able to take a look at this, or suggest a more appropriate reviewer?

This was uncovered while working on #220316. That change starts emitting previously dropped OpenMP host diagnostics, which exposed that the requires state is lost across PCH/module serialization. This PR fixes that serialization issue and adds regression tests for both PCH and modules.

https://github.com/llvm/llvm-project/pull/220058