Frontend: Define __EXCEPTIONS if -fexceptions is passed

GCC defines __EXCEPTIONS, regardless of language mode, if -fexceptions
is passed.  We should do the same.

This fixes PR21358.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220714 91177308-0d34-0410-b5e6-96231b3b80d8