Add returnValue scope to DAP

The DAP spec recently changed to add a new scope for the return value
from a "stepOut" request.  This new scope uses the "returnValue"
presentation hint.  See:

    https://github.com/microsoft/debug-adapter-protocol/issues/458

This patch implements this for gdb.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31945
Reviewed-By: Eli Zaretskii <eliz@gnu.org>