Improve UnwindLLDB with better detection for unwinding failures

Previously we accepted a frame as correct result if the PC pointed
into an executable section of code. The isse with that approac is
that if we calculated PC correctly but messed up the value of CFA
then unwinding from the next fram will most likely fail.

With this change I modify the logic with keeping the requirement
for PC to point to an executable section and also check that we can
continue the unwind from the frame we calculated. If continuing from
the frame calculated with the primary unwind plan isn't working then
fall back to the fallback plan with the hope for a better frame (if
the fallback plan won't help then we acceot the frame from the
primary plan).

Differential revision: http://reviews.llvm.org/D10932

llvm-svn: 241434