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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@70273 91177308-0d34-0410-b5e6-96231b3b80d8