[clang] [clang] Rename diag notes that assumed precompiled dependencies are pch's, NFCI (PR #142161)

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

I think in the second case we usually use "AST file" to lump together .pcm and .pch.

https://github.com/llvm/llvm-project/pull/142161