Add "support" for DW_CFA_GNU_args_size to the unwinder

Summary:
This adds the knowledge of the DW_CFA_GNU_args_size instruction to the eh_frame parsing code.
Right now it is ignored as I am unsure how is it supposed to be handled, but now we are at least
able to parse the rest of the FDE containing this instruction.

I also add a fix for a bug which was exposed by this instruction. Namely, a mismatched sequence
of remember/restore instructions in the input could cause us to pop an empty stack and crash. Now
we just log the error and ignore the offending instruction.

Reviewers: jasonmolenda

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D22266

llvm-svn: 275260