Ada: ada_to_fixed_value should also unwrap_value

Ada: ada_to_fixed_value should also unwrap_value

We should always unwrap a value before trying to "fix" it. It is
therefore logical that ada_to_fixed_value would call unwrap_value
before creating the fixed value.

This simplifies the code in ada-lang.c a bit.

gdb/ChangeLog:

        * ada-lang.c (ada_to_fixed_value): Call unwrap_value before
        creating fixed value.
        (ada_value_ind, ada_coerce_ref, assign_component)
        (ada_evaluate_subexp): Remove call to unwrap_value before
        call to ada_to_fixed_value.