[DebugInfo] Support zero-length CIE in the _eh_frame parser

MCJIT emits zero-length CIE at the end of the _eh_frame section. This change
ensures that parser inside DebugInfo will not crash and correctly record such cases.
We are now recording DW_EH_PE_omit as a default value for FDE and LSDA encodings.
Also Offset != EndAugmentationOffset assertion check will only happen if augmentation 
string had 'z' letter in it.

Differential Revision: http://reviews.llvm.org/D16588

llvm-svn: 258931