RegisterContextLLDB::InitializeNonZerothFrame had a bit of code to
detct unwind loops but there was a code path through there (using
architecture default unwind plans) that didn't do the check, and
could end up with an infinite loop unwind.  Move that code into a
separate method and call it from both places where it is needed.

Also remove the use of ABI::FunctionCallsChangeCFA in that check.
I thought about it a lot and none of the architecutres that we're
supporting today can have a looping CFA.

Since the unwinder isn't using ABI::FunctionCallsChangeCFA() and
ABI::StackUsesFrames(), and the unwinder was the only reason
those methods exists, I removed them from the ABI and all its
plugins.

<rdar://problem/17364005>