[clang-cl] Disable C++ exceptions a different way

Rather than making -fexceptions a core option that enables C++ EH in
clang-cl, users can use the '-Xclang -fexceptions -Xclang
-fcxx-exceptions' flag set. We weren't going to expose -fexceptions in
clang-cl in the long run, so this way we don't add and then remove a
flag.

llvm-svn: 242176