Partially revert Doug's PCH validation patch (r98585).

Partially revert Doug's PCH validation patch (r98585).

This patch completely defeated the "passing in a prestat'd size
to MemoryBuffer" optimization, leading to an extra fstat call for
every buffer opened, in order to find out if the datestamp and size
of the file on disk matches what is in the stat cache.

I fully admit that I don't completely understand what is going on here:
why punish code when a stat cache isn't in use?  what is the point of a
stat cache if you have to turn around and stat stuff to validate it?

To resolve both these issues, just drop the modtime check and check the
file size, which is the important thing anyway.  This should also resolve
PR6812, because presumably windows is stable when it comes to file sizes.
If the modtime is actually important, we should get it and keep it on the
first stat.

This eliminates 833 fstat syscalls when processing Cocoa.h, speeding up
system time on -Eonly Cocoa.h  from 0.041 to 0.038s.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@120001 91177308-0d34-0410-b5e6-96231b3b80d8