[clang] Improve module out of date error message (#128103)
When a pcm file has a different size or modification time than it had
when it was written to another module's IMPORT table Clang emits:
`<pcm> is out of date and needs to be rebuilt: module file out of date`
This is difficult to understand what's happening because there are a lot
of reasons that a module file can be out of date. This changes the
latter part of that message to:
`module file has a different size or mtime than expected`
Which makes it clearer what the issue is. For future work it would be
nice if a more detailed explanation of the issue could be emitted as a
note instead.
