Optimize the preprocessor's handling of the __import_module__

keyword. We now handle this keyword in HandleIdentifier, making a note
for ourselves when we've seen the __import_module__ keyword so that
the next lexed token can trigger a module import (if needed). This
greatly simplifies Preprocessor::Lex(), and completely erases the 5.5%
-Eonly slowdown Argiris noted when I originally implemented
__import_module__. Big thanks to Argiris for noting that horrible
regression!

llvm-svn: 139265