Add support for the llvm.unwind intrinsic, which we codegen to just do an abort

until we implement unwinding.
Add support for the invoke instruction, which codegens just like a call with
a branch after it.

The end effect of this change is that programs using the invoke instruction,
but never unwinding, will work fine.  Programs that unwind will abort until
we get unwind support.

llvm-svn: 8187