[LLVM-C] Add more debug information accessors to GlobalObject and Instruction

Summary: Adds missing debug information accessors to GlobalObject.  This puts the finishing touches on cloning debug info in the echo tests.

Reviewers: whitequark, deadalnix

Reviewed By: whitequark

Subscribers: aprantl, JDevlieghere, llvm-commits, harlanhaskins

Differential Revision: https://reviews.llvm.org/D51522

llvm-svn: 343330