[clang][modules] Derive mtime from PCM timestamps, not PCM files (#162965)
#137363 was supposed to be NFC for the `CrossProcessModuleCache` (a.k.a
normal implicit module builds), but accidentally passed the wrong path
to `sys::fs::status`. Then, #141358 removed the correct path that
should've been passed instead. (The variable was flagged as unused.)
None of our existing tests caught this regression, we only found out due
to a SourceKit-LSP benchmark getting slower.
This PR re-implements the original behavior, adds new remark to Clang
for PCM input file validation, and uses it to create more reliable tests
of the `-fmodules-validate-once-per-build-session` flag.
llvm-project
ce8abef2 - [clang][modules] Derive mtime from PCM timestamps, not PCM files (#162965)
