LanguageRuntime can provide an UnwindPlan for special occasions

Add a facility in the LanguageRuntime to provide a special
UnwindPlan based on the register values in a RegisterContext,
instead of using the return-pc to find a function and use its
normal UnwindPlans.

Needed when the runtime has special stack frames that we want
to show the user, but aren't actually on the real stack.
Specifically for Swift asynchronous functions.

With feedback from Greg Clayton, Jonas Devlieghere, Dave Lee

<rdar://problem/70398009>

Differential Revision: https://reviews.llvm.org/D96839