Introduce cp_print_value_fields and c_value_print_struct

Introduce cp_print_value_fields and c_value_print_struct

This adds cp_print_value_fields and c_value_print_struct, value-based
analogues of the corresponding val-printing functions.  Note that the
Modula-2 printing code also calls cp_print_val_fields, and so is
updated to call the function function.

gdb/ChangeLog
2020-03-13  Tom Tromey  <tom@tromey.com>

	* m2-valprint.c (m2_value_print_inner): Use
	cp_print_value_fields.
	* cp-valprint.c	(cp_print_value_fields): New function.
	* c-valprint.c (c_value_print_struct): New function.
	(c_value_print_inner): Use c_value_print_struct.
	* c-lang.h (cp_print_value_fields): Declare.