[EH] Use __CxxFrameHandler3 for C++ EH in MS environments

Fixes regression introduced by r319297. MSVC environments still use SEH
unwind opcodes but they should use the Microsoft C++ EH personality, not
the mingw one.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@319363 91177308-0d34-0410-b5e6-96231b3b80d8