re PR c++/20145 (template "class has virtual functions ... " is not suppressed with -isystem)

	* cp-tree.def (TINST_LEVEL): Document TINST_IN_SYSTEM_HEADER_P.
	* cp-tree.h (struct tinst_level): Add in_system_header_p.
	(TINST_IN_SYSTEM_HEADER_P): New macro.
	(make_tinst_level): Remove.
	* pt.c (lookup_template_class): Preserve DECL_IN_SYSTEM_HEADER on
	the instantiated class.
	(push_tinst_level): Do not use make_tinst_level.  Set
	TINST_IN_SYSTEM_HEADER_P.
	(pop_tinst_level): Likewise.
	(instantiate_class_template): Set in_system_header.
	(instantiate_pending_templates): Likewise.
	* tree.c (make_tinst_level): Remove.

	PR c++/20145
	* g++.dg/warn/Wdtor1.C: New test.

From-SVN: r97854