Return global scope from DAP scopes request

Return global scope from DAP scopes request

A co-worker requested that the DAP code emit a scope for global
variables.  It's not really practical to do this for all globals, but
it seemed reasonable to do this for globals coming from the frame's
compilation unit.  For Ada in particular, this is convenient as it
exposes package-scoped variables.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>