This is a follow-up to r62675:

Refactor how the preprocessor changes a token from being an tok::identifier to a 
keyword (e.g. tok::kw_for).  Instead of doing this in HandleIdentifier, hoist this
common case out into the caller, so that every keyword doesn't have to go through
HandleIdentifier.  This drops time in HandleIdentifier from 1.25ms to .62ms, and
speeds up clang -Eonly with PTH by about 1%.

llvm-svn: 62855