Update RegisterContextLLDB::GetFullUnwindPlanForFrame() to use the architectural-
default-at-first-instruction UnwindPlan if we're at the beginning of a function and
the ABI can provide us with an UnwindPlan to get out of there before falling back
to the generic architectural default UnwindPlan (which usually assumes that the stack
has already been set up.)

Update the FuncUnwinders methods to gracefully handle the case where an assembly
profiler may not be available.

Fix a bug where FuncUnwinders::GetUnwindPlanArchitectureDefaultAtFunctionEntry was
returning the wrong UnwindPlan to its caller.