Use SEARCH_FUNCTION_DOMAIN when looking for Ada exception symbols

While working on another bug, I noticed that the Ada code to find
exception symbols uses SEARCH_VFT.  This will find variables and types
-- but only functions are needed here.  This patch changes the code to
use SEARCH_FUNCTION_DOMAIN.

Tested on x86-64 Fedora 38, using a version of GNAT with the debuginfo
installed, to ensure the exception-related tests work.

Reviewed-by: Keith Seitz <keiths@redhat.com>