[clang] [clang][modules] Minor improvements to diagnosing `out of date` errors (PR #136612)

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

Does the described test case look like a build system problem? In my understanding at step 4 we should have re-scanned B to know that we need some B'.pcm depending on A-2.pcm. Regardless of the answer I don't want to block the change as it is still an improvement. Just thinking if end users are even supposed to see the new diagnostic. And if it should be actionable for the end users.

https://github.com/llvm/llvm-project/pull/136612