Version N of the llvm_unreachable patch: VC++ doesn't recognize that abort()

Version N of the llvm_unreachable patch:  VC++ doesn't recognize that abort()
doesn't return, so just go back to using the old runtime function instead
of trying to use abort() when __builtin_unreachable (or an equivalent) isn't
supported.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127629 91177308-0d34-0410-b5e6-96231b3b80d8