Rename objfile_type to builtin_type

This renames objfile_type to be an overload of builtin_type, in
preparation for their unification.

Reviewed-By: Simon Marchi <simon.marchi@efficios.com>

gdb/gdbtypes.c

@@ -6084,7 +6084,7 @@ static const registry<objfile>::key<struct objfile_type,
   objfile_type_data;
 
 const struct objfile_type *
-objfile_type (struct objfile *objfile)
+builtin_type (struct objfile *objfile)
 {
   struct gdbarch *gdbarch;
   struct objfile_type *objfile_type = objfile_type_data.get (objfile);