[clang][modules] Avoid unnecessary writes of .timestamp files

Clang currently updates the mtime of .timestamp files on each load of the corresponding .pcm file. This is not necessary. In a given build session, Clang only needs to write the .timestamp file once, when we first validate the input files. This patch makes it so that we only touch the .timestamp file when it's older than the build session, alleviating some filesystem contention in clang-scan-deps.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D149802