Have ptype handle types declared pragma Unchecked_Variants.

Have ptype handle types declared pragma Unchecked_Variants.

When a type is marked with pragma Unchecked_Variants, ptype did not print
variants having a single component, since the compiler produces incorrect
debugging output for such cases.  With this patch, we special-case these
components so that they print.

Changelog:

    * gdb/ada-typeprint.c (print_selected_record_field_types): New function,
    incorporating and generalizing print_record_field_types.
    (print_record_field_types): Change return value and update comment.
    Re-implement using print_selected_record_field_types.
    (print_choices): Print "=>" here.
    Handle case of unencoded variant branch.
    (print_variant_clauses): Reformat comment.
    Special-case unencoded variant branch.