(Ada) New function ada_is_access_to_unconstrained_array

Add a new function to check if a given type is an access to an
unconstrained array. This function contains code that is present only
once in the current sources but will be used in a future patch.

gdb/ChangeLog:

        * ada-lang.c (ada_is_access_to_unconstrained_array): New function.
        (ada_check_typedef): Use it.

Tested on x86_64-linux.