struct block parameter constification in ada-lang.c

struct block parameter constification in ada-lang.c

This patch series constifies a number of struct block * parameters.

gdb/ChangeLog:

        * ada-lang.c (ada_add_block_symbols, add_defn_to_vec)
        (lookup_cached_symbol, ada_add_local_symbols): Add "const" to
        struct block * parameter.
        (ada_lookup_symbol_list_worker): Constify local variable "block".
        Remove cast which is no longer necessary.