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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@242176 91177308-0d34-0410-b5e6-96231b3b80d8