Build up UnwindPlan::PlanValidAtAddress to recognize some general
indications that the UnwindPlan is invalid -- for instance, a
complete lack of rows, or a row that fails to define a register to
base the CFA off of.
<rdar://problem/15246247> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@196201 91177308-0d34-0410-b5e6-96231b3b80d8