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

[Volodymyr Sapsai via cfe-commits]

https://github.com/vsapsai approved this pull request.

Discussed it with Cyndy more and I think that the trade-off is
* "AST file" is easier to search for as it is more specific
* "AST file" can be more confusing for non-compiler developers as they aren't required to work with and to know about AST

So even if "AST file" is technically correct (the best kind of correct), I don't think it is worth using it in a public-facing diagnostics (still fine to keep in constants like `note_ast_file_required_by`).

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