CodeGen: make a check stricter

When targeting Windows itanium (a MSVC environment), use itanium style
exceptions rather than SEH.  Existing test cases already test this code path.
Applying this change ensures that tests wont break due to a parallel change in
LLVM (to correctly report isMSVCEnvironment).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222179 91177308-0d34-0410-b5e6-96231b3b80d8