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