[clang] [clang][OpenMP] Emit host diagnostics that were deferred and never flushed (PR #220316)

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pgerell wrote:

@alexey-bataev Unfortunately the drift of generated tests on main didn't fully explain the large diff here.
It was mainly due to changed emission order in this PR.
I did put the largest regenerations as separate commits to simplify review.


https://github.com/llvm/llvm-project/pull/220316