c++: improve print_node of PTRMEM_CST

It's been inconvenient that pretty-printing of PTRMEM_CST didn't display
what member the constant refers to.

Adding that is complicated by the absence of a langhook for CONSTANT_CLASS_P
nodes; the simplest fix for that is to use the tcc_exceptional hook for
tcc_constant as well.

gcc/cp/ChangeLog:

	* ptree.c (cxx_print_xnode): Handle PTRMEM_CST.

gcc/ChangeLog:

	* langhooks.h (struct lang_hooks): Adjust comment.
	* print-tree.c (print_node): Also call print_xnode hook for
	tcc_constant class.