Introduce c_value_print_memberptr

Introduce c_value_print_memberptr

This adds c_value_print_memberptr, a value-based analogue of
c_val_print_memberptr.

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

	* c-valprint.c (c_value_print_memberptr): New function.
	(c_value_print_inner): Use it.