The default of the dispatch switch statement was to branch to a BB that executed

The default of the dispatch switch statement was to branch to a BB that executed
the 'unwind' instruction. However, later on that instruction was converted into
a jump to the basic block it was located in, causing an infinite loop when we
get there.

It turns out, we get there if the _Unwind_Resume_or_Rethrow call returns (which
it's not supposed to do). It returns if it cannot find a place to unwind
to. Thus we would get what appears to be a "hang" when in reality it's just that
the EH couldn't be propagated further along.

Instead of infinitely looping (or calling `unwind', which none of our back-ends
support (it's lowered into nothing...)), call the @llvm.trap() intrinsic
instead. This may not conform to specific rules of a particular language, but
it's rather better than infinitely looping.

<rdar://problem/9175843&9233582>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@129302 91177308-0d34-0410-b5e6-96231b3b80d8