Tweak SysV_arm64 function entry unwind plan

[labath]

Summary

The motivation for this is to make sure the first row of the plan
compares equal to the first row of a generic debug_frame unwind plan.
Right now, the code in FuncUnwinders::GetUnwindPlanAtNonCallSite
considers them unequal because they specify the return address in a
different way (SetReturnAddressRegister(LR) vs. an explicit PC=LR rule).
This means that FuncUnwinders would always choose the debug_frame unwind
plan, which is not correct, as that one is usually not correct at all
locations.

Right now this is basically a noop because we don't have parse any
debug_frame plans, but it fixes some test failures when merging D33504
in.

I have to say I don't understand the full implications of the switch to
SetReturnAddressRegister() way of doing things, but given that all of
our other unwind plans (eh_frame, instruction profiling) do it this way,
it sounds like the right thing to do.

[tberghammer]

Looks good

[jasonmolenda]

Looks good to me.

[omjavaid]

LGTM

lldb/trunk/source/Plugins/ABI/SysV-arm64/ABISysV_arm64.cpp

 }
 
 bool ABISysV_arm64::CreateFunctionEntryUnwindPlan(UnwindPlan &unwind_plan) {
   unwind_plan.Clear();
   unwind_plan.SetRegisterKind(eRegisterKindDWARF);
 
   uint32_t lr_reg_num = arm64_dwarf::lr;
   uint32_t sp_reg_num = arm64_dwarf::sp;
-  uint32_t pc_reg_num = arm64_dwarf::pc;
 
   UnwindPlan::RowSP row(new UnwindPlan::Row);
 
   // Our previous Call Frame Address is the stack pointer
   row->GetCFAValue().SetIsRegisterPlusOffset(sp_reg_num, 0);
 
-  // Our previous PC is in the LR
-  row->SetRegisterLocationToRegister(pc_reg_num, lr_reg_num, true);
-
   unwind_plan.AppendRow(row);
+  unwind_plan.SetReturnAddressRegister(lr_reg_num);
 
   // All other registers are the same.
 
   unwind_plan.SetSourceName("arm64 at-func-entry default");
   unwind_plan.SetSourcedFromCompiler(eLazyBoolNo);
+  unwind_plan.SetUnwindPlanValidAtAllInstructions(eLazyBoolNo);
 
   return true;
 }
 
 bool ABISysV_arm64::CreateDefaultUnwindPlan(UnwindPlan &unwind_plan) {
   unwind_plan.Clear();
   unwind_plan.SetRegisterKind(eRegisterKindDWARF);