Remove -fseh-exceptions in favor of checking the triple

This option was misleading because it looked like it enabled the
language feature of SEH (__try / __except), when this option was really
controlling which EH personality function to use. Mingw only supports
SEH and SjLj EH on x86_64, so we can simply do away with this flag.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221963 91177308-0d34-0410-b5e6-96231b3b80d8