[clang-cl] Make /EHs turn on C++ EH once again

C++ exceptions are still off by default, which is similar to how C++
cleanups are off by default in MSVC.

If you use clang instead of clang-cl, exceptions are also still off by
default. In the future, when C++ EH is proven to be stable, we may flip
the default for that driver to be consistent with other platforms.

llvm-svn: 249704