Lex: Fix __has_feature(cxx_exceptions) for objective C++

At one point, -fexceptions was a synonym for -fcxx-exceptions. While
the driver options still enables cxx-exceptions by default, the cc1
flag is purely about exception tables and this doesn't account for
objective C exceptions. Because of this, checking for the
cxx_exceptions feature in objective C++ often gives the wrong answer.

The cxx_exceptions feature should be based on the -fcxx-exceptions cc1
flag, not -fexceptions. Furthermore, at some point the tests were
changed to use cc1 even though they were testing the driver behaviour.
We're better off testing both the driver and cc1 here.