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

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

I think this will slow down scanning. The race is rare, but this check will be performed for every module compile. At least in the scanner, two racing threads should produce the same module file, so I'd rather investigate the discrepancy in the produced PCMs (unless the only bug present is the one I fix in https://github.com/llvm/llvm-project/pull/194888).

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