* semantics.c (call_stack, call_stack_tick, cx_error_context): New.
	(last_cx_error_tick, push_cx_call_context, pop_cx_call_context): New.
	(cxx_eval_call_expression): Call push/pop_cx_call_context instead
	of giving follow-on errors.
	* error.c (maybe_print_constexpr_context): New.
	(cp_diagnostic_starter): Call it.
	* cp-tree.h: Declare cx_error_context.

From-SVN: r166169