Add return value to DAP scope

A bug report in the DAP specification repository pointed out that it
is typical for DAP implementations to put a function's return value
into the outermost scope.

This patch changes gdb to follow this convention.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31341
Reviewed-By: Kévin Le Gouguec <legouguec@adacore.com>