Add a -fcxx-exceptions flag to the frontend, which can be used to enable

C++ exceptions, even when exceptions have been turned off using -fno-exceptions.
Make the -fobjc-exceptions flag do the same thing, but for Objective-C exceptions.

C++ and Objective-C exceptions can also be disabled using -fno-cxx-excptions and
-fno-objc-exceptions.

llvm-svn: 126630