[clang] [clang][modules] Erase PCMs not meeting expectations from in-memory cache (PR #177004)

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

> I have an alternative fix at #176537 I like how you handle other cases with pre-existing buffers and not just those loaded from disk. Need to think more if there are more problems possible when using those pre-existing buffers.

After going through the other cases for pre-existing buffers it doesn't look like we would be able to call `erasePCM` anyway. Specifically, for `Modules.lookup` the found Module should be in the final state, otherwise we wouldn't even be looking for such module. And the result of `lookupBuffer` we put into `InMemoryModuleCache` in the finalized state.

https://github.com/llvm/llvm-project/pull/177004