Don't claim exception related arguments when looking at RTTIMode

Summary:
We were claiming the -f*exceptions arguments when looking for the
RTTIMode. This makes us not warn about unused arguments if compiling a C
file with -fcxx-exceptions.

This patch fixes it by not claiming the exception-related arguments at
that point.

Reviewers: rsmith, samsonov

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D8507

llvm-svn: 232860