Ada exception catchpoint support cleanup.

Ada exception catchpoint support cleanup.

This patch cleans up a bit the way we detect which type of runtime
the program uses with respect to Ada exceptions. It also removes
an unnecessary check in ada_exception_sal which is already performed
by ada_exception_support_info_sniffer.

Some of the changes are preparation work for detecting the situation
where the Ada runtime is found, but lacking debugging info.

gdb/ChangeLog:

        * ada-lang.c (ada_has_this_exception_support): New function,
        extracted out of ada_exception_sal and ada_exception_sal.
        (ada_exception_support_info_sniffer): Simplify by using
        ada_has_this_exception_support.
        (ada_exception_sal): Replace unnecessary checks by assertions.
        Minor simplifications.