Enhance FuncUnwinders::GetUnwindPlanAtNonCallSite to detect when we

may be in a function that is non-ABI conformant, and the eh_frame
instructions correctly describe how to unwind out of this function,
but the assembly parsing / arch default unwind plans would be 
incorrect.

This is to address a problem that Ravitheja Addepally reported in
http://reviews.llvm.org/D21221 - I wanted to try handling the problem
with this approach which I think may be more generally helpful, 
Ravitheja tested it and said it solves the problem on Linux/FreeBSD.
Ravi has a test case in http://reviews.llvm.org/D21221 that will
be committed separately.

Thanks for all the help on this one, Ravi.

llvm-svn: 274700