[PCH] Fix timestamp check on windows hosts.

On Linux, if the timestamp of a header file, included in the pch, is modified, then including the pch without regenerating it causes a fatal error, which is reasonable.
On Windows the check is ifdefed out, allowing the compilation to continue in a broken state.
The root of the broken state is that, if timestamps dont match, the preprocessor will reparse a header without discarding the pch data.
This leads to "#pragma once" header to be included twice.
The reason behind the ifdefing of the check lacks documentation, and was done 6 years ago.
This change tentatively removes the ifdefing.

First part of patch proposed at:

Differential Revision: http://reviews.llvm.org/D20867

llvm-svn: 275261