Back out r221229 -- instead of trying to identify the end of the unwind,
let's let lldb try the arch default unwind every time but not destructively --
it doesn't permanently replace the main unwind method for that function from
now on.

This fix is for <rdar://problem/18683658>.  

I tested it against Ryan Brown's go program test case and also a
collection of core files of tricky unwind scenarios 
<rdar://problem/15664282> <rdar://problem/15835846>
<rdar://problem/15982682> <rdar://problem/16099440>
<rdar://problem/17364005> <rdar://problem/18556719> 
that I've fixed over the last 6-9 months.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@221238 91177308-0d34-0410-b5e6-96231b3b80d8
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
Add recognition for another x86 epilogue sequence (ret followed by
a nop).  Fixes an instruction stepping problem when trying to step
over the final instructions of an epilogue.
<rdar://problem/18068877> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@221241 91177308-0d34-0410-b5e6-96231b3b80d8