[clang] [clang][deps] Fix race condition in implicit Clang module builds (PR #194757)

[Naveen Seth Hanig via cfe-commits]

naveen-seth wrote:

If I understand correctly, we’d remove the assert that motivated this PR and only detect any discrepancies from duplicate‑built PCMs?

Otherwise, could we do a cheaper check by storing the build state in `ModuleCacheEntry`?
(After locking, we'd then check the state).

https://github.com/llvm/llvm-project/pull/194757