[LLVM-C] Add Bindings to Access an Instruction's DebugLoc

Summary: Provide direct accessors to supplement LLVMSetInstDebugLocation.  In addition, properly accept and return the NULL location.  The old accessors provided no way to do this, so the current debug location cannot currently be cleared.

Reviewers: whitequark, deadalnix

Reviewed By: whitequark

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60481

llvm-svn: 358038