Get rid of some useless uses of NoExtensions. The philosophy here is

that if we're going to print an extension warning anyway, 
there's no point to changing behavior based on NoExtensions: it will 
only make error recovery worse.

Note that this doesn't cause any behavior change because NoExtensions 
isn't used by the current front-end.  I'm still considering what to do about
the remaining use of NoExtensions in IdentifierTable.cpp.

llvm-svn: 70273