Debug Info: move DIScope::getContext back from DwarfDebug.

This partially reverts r190330. DIScope::getContext now returns DIScopeRef
instead of DIScope. We construct a DIScopeRef from DIScope when we are
dealing with subprogram, lexical block or name space.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190362 91177308-0d34-0410-b5e6-96231b3b80d8