Make fixed_point_type_base_type a method of struct type

Make fixed_point_type_base_type a method of struct type

As suggested by Simon, to logically connect this function to
the object it inspects.

Note that, logically, this method should be "const". Unfortunately,
the implementation iterates on struct type objects starting with "this",
and thus trying to declare the method "const" triggers a compilation
error.

gdb/ChangeLog:

        * gdbtypes.h (struct type) <fixed_point_type_base_type> New method,
        replacing the fixed_point_type_base_type function. All callers
        updated throughout this project.
        (fixed_point_type_base_type): Remove declaration.
        * gdbtypes.c (type::fixed_point_type_base_type): Replaces
        fixed_point_type_base_type.  Adjust implementation accordingly.