c++: import/export NTTP objects

c++: import/export NTTP objects

This adds smarts to the module machinery to handle NTTP object
VAR_DECLs.  Like typeinfo objects, these must be ignored in the symbol
table, streamed specially and recreated on stream in.

	gcc/cp/
	PR c++/100616
	* module.cc (enum tree_tag): Add tt_nttp_var.
	(trees_out::decl_node): Handle NTTP objects.
	(trees_in::tree_node): Handle tt_nttp_var.
	(depset::hash::add_binding_entry): Skip NTTP objects.

	gcc/testsuite/
	PR c++/100616
	* g++.dg/modules/100616_a.H: New.
	* g++.dg/modules/100616_b.C: New.
	* g++.dg/modules/100616_c.C: New.
	* g++.dg/modules/100616_d.C: New.