Debug Info: In DIBuilder, the context field of a global variable is updated to
use DIScopeRef.

A paired commit at clang will follow to show cases where we will use an
identifer for the context of a global variable.

rdar://18958417


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222195 91177308-0d34-0410-b5e6-96231b3b80d8