[LLVM-C] Add Accessors For Global Variable Metadata Properties

Summary: Metadata for a global variable is really a (GlobalVariable, Expression) tuple. Allow access to these, then allow retrieving the file, scope, and line for a DIVariable, whether global or local. This should be the last of the accessors required for uniform access to location and file information metadata.

Reviewers: jberdine, whitequark, deadalnix

Reviewed By: jberdine, whitequark

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60725

llvm-svn: 358532