Add a test to distinguish between reserved tokens and normal identifiers.

The -fms-extensions option affects a number of subtle front-end C/C++
behaviors, and it would be useful to be able to distinguish MS keywords
from regular identifiers in the ms-extensions mode even if the triple
does not define a Windows target. It should make life easier if anyone
needs to port their Windows codes to elsewhere.

Differential Revision: http://reviews.llvm.org/D3034