ada-lang.c: Renaming some fixed-point-related routines

ada-lang.c: Renaming some fixed-point-related routines

This patch renames some of the fixed-point-related subprograms in ada-lang.c
so as to make it obvious that those routines only handle the case where
the types are encoded using the GNAT encoding.

No function change; this patch is preparation work for adding support
for fixed-point types purely based on standard DWARF debug info.

gdb/ChangeLog:

        * ada-lang.c (cast_from_gnat_encoded_fixed_point_type): Renames
        cast_from_fixed.  Update all callers.
        (cast_to_gnat_encoded_fixed_point_type): Renames cast_to_fixed.
        Update all callers.
        (gnat_encoded_fixed_point_scaling_factor): Renames ada_scaling_factor.
        Update all callers.
        * ada-lang.h (gnat_encoded_fixed_point_scaling_factor): Renames
        ada_scaling_factor.
        * ada-typeprint.c: Replace call to ada_scaling_factor by call
        to print_gnat_encoded_fixed_point_type.
        * ada-valprint.c: Likewise.