Add one extra sanity check to RegisterContextLLDB::TryFallbackUnwindPlan
so it doesn't try the arch default if a comiler-generated (eh_frame,
compact unwind info) based unwind plan has failed.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@221239 91177308-0d34-0410-b5e6-96231b3b80d8