Use skip_spaces and skip_to_space in ada-lang.c

Use skip_spaces and skip_to_space in ada-lang.c

This is just a minor cleanup that reduces a bit the code in ada-lang.c
by using skip_spaces or skip_to_space, instead of doing the skipping
ourselves.

gdb/ChangeLog:

        * ada-lang.c: #include "cli/cli-utils.h".
        (get_selections): Use skip_spaces.
        (ada_get_next_arg): Use skip_spaces and skip_to_space.
        (catch_ada_exception_command_split): Use skip_spaces.
        (ada_decode_assert_location): Likewise.