[LLVM-C] Correct The Current Debug Location Accessors

Summary: Deprecate the existing accessors for the "current debug location" of an IRBuilder.  The setter could not handle being reset to NULL, and the getter would create bogus metadata if the NULL location was returned.  Provide direct metadata-based accessors instead.

Reviewers: whitequark, deadalnix

Reviewed By: whitequark

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60484

llvm-svn: 358039