support operator keywords used in Windows SDK

to support operator keywords used in Windows SDK, alter token type when 
seen in system headers

Hello, I submitted D33505 to address this problem, but the 
proposal was rejected as too big a hammer.
This change will allow clang to parse the WindowsSDK header <query.h> 
which uses the operator name "or" as a field name. Treat cpp operator 
keywords as ordinary identifiers inside the Microsoft headers, but 
treat them as usual in the user's program.

Original Submitter: Melanie Blower (mibintc)

Differential Revision: https://reviews.llvm.org/D33782

llvm-svn: 305087