[Ada] Do not re-cache symbol-lookup result found from cache lookup.

[Ada] Do not re-cache symbol-lookup result found from cache lookup.

When ada-lang.c:ada_lookup_symbol_list_worker finds a match in
the symbol cache, it caches the result again, which is unecessary.
This patch fixes the code to avoid that.

gdb/ChangeLog:

        PR gdb/17856:
        * ada-lang.c (ada_lookup_symbol_list_worker): Do not re-cache
        results found in the cache.

Tested on x86_64-linux, no regression.