[DebugInfo] Change default value of FDEPointerEncoding

Summary:
If the encoding is not specified in CIE augmentation string, then it
should be DW_EH_PE_absptr instead of DW_EH_PE_omit.

Reviewers: ruiu, MaskRay, plotfi, rafauler

Reviewed By: MaskRay

Subscribers: rafauler, JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D49000

llvm-svn: 336577