Introduce generic_value_print

Introduce generic_value_print

This introduces generic_value_print, which is a value-based analogue
to generic_val_print.  For now this is unused and simply calls
generic_val_print, but subsequent patches will both change this
function to work using the value API directly, and convert callers of
generic_val_print to call this instead.

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

	* valprint.h (generic_value_print): Declare.
	* valprint.c (generic_value_print): New function.