Reapply r205655, DebugInfo: Place global constants in their appropriate context.

This was reverted in 205664 and seems to be fixed by 205668... though
that may be more by accident than anything well founded.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205669 91177308-0d34-0410-b5e6-96231b3b80d8