Add a bit to IdentifierInfo that acts as a simple predicate which

Add a bit to IdentifierInfo that acts as a simple predicate which
tells us whether Preprocessor::HandleIdentifier needs to be called.
Because this method is only rarely needed, this saves a call and a
bunch of random checks.  This drops the time in HandleIdentifier
from 3.52ms to .98ms on cocoa.h on my machine.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@62675 91177308-0d34-0410-b5e6-96231b3b80d8