Emit an error when attempting to generate IR for SEH __try

Currently we silently omit the code in the try and finally bodies, which
is pretty bad.  This way we fail loudly.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190809 91177308-0d34-0410-b5e6-96231b3b80d8