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

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

Right now I don't have a strong opinion about the specific terminology. Though it might change as I think about it more.

What I do have a strong opinion about is mentioning a specific file. In the first quick pass it looks like we do that already, e.g., "please rebuild precompiled file **'%0'**".

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