Debug Info: move DIScope::getContext to DwarfDebug.

DIScope::getContext is a wrapper function that calls the specific getContext
method on each subclass. When we switch DIType::getContext to return DIScopeRef
instead of DIScope, DIScope::getContext can no longer return a DIScope without
a type identifier map.

DIScope::getContext is only used by DwarfDebug, so we move it to DwarfDebug
to have easy access to the type identifier map.