Refactor val_print and common_val_print

Refactor val_print and common_val_print

This changes val_print and common_val_print to use a new helper
function.  A theme in the coming patches is that calls to val_print
itself should be removed.  This is the first such patch; at the end of
the series, we'll remove val_print and simplify do_val_print.

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

	* valprint.c (do_val_print): New function, from val_print.
	(val_print): Use do_val_print.
	(common_val_print): Use do_val_print.