[Ada] Add a symbol lookup cache

[Ada] Add a symbol lookup cache

This patch implements the caching mechanism alluded to in a comment
next to some stubbed functions.

gdb/ChangeLog:

        * ada-lang.c (HASH_SIZE): New macro.
        (struct cache_entry): New type.
        (cache_space, cache): New static globals.
        (ada_clear_symbol_cache, find_entry): New functions.
        (lookup_cached_symbol, cache_symbol): Implement.
        (ada_new_objfile_observer, ada_free_objfile_observer): New.
        (_initialize_ada_language): Attach ada_new_objfile_observer
        and ada_free_objfile_observer.