update name of several Ada fixed-point type handling functions

update name of several Ada fixed-point type handling functions

The purpose of this patch is to prepare for the future where
fixed point types become described using standard DWARF info,
rather than GNAT encodings. For that, we rename a number of
routines manipulating Ada fixed point types to make it explicit
from their new names that they rely on the GNAT encodings to work.
This will allow us, when we introduce support for fixed point types
from standard DWARF to use names that are not ambiguous with
the functions that do similar work, but only for GNAT encodings.

gdb/ChangeLog:

        * ada-lang.h: (ada_is_gnat_encoded_fixed_point_type): Renames
        ada_is_fixed_point_type.  Update all callers.
        (gnat_encoded_fixed_point_delta): Renames ada_delta.  Update
        all callers.
        * ada-lang.c (gnat_encoded_fixed_type_info): Renames fixed_type_info.
        Update all callers.
        * ada-typeprint.c (print_gnat_encoded_fixed_point_type): Renames
        print_fixed_point_type.  Update all callers.
        * ada-valprint.c (ada_value_print_num): Replace call to
        ada_is_fixed_point_type by ada_is_gnat_encoded_fixed_point_type.