[clang] [clang][modules][deps] Optimize in-process timestamping of PCMs (PR #137363)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

(Just adding a note here for my records that this speeds up from-scratch scans by 1.4% and incremental _null_ scans by 67% on my benchmark.)

https://github.com/llvm/llvm-project/pull/137363