[clang] [clang][modules] Write the module timestamp once per build session (PR #210587)

[Taimuraz Kaitmazov via cfe-commits]

atassis wrote:

I opened this PR because of a flaky test. I was pulled into the blamelist [here](https://lab.llvm.org/buildbot/#/builders/4/builds/12487) for a failure that turned out to be this race, not my change


https://github.com/llvm/llvm-project/pull/210587