c++: computed goto warning [PR37722]

PR37722 complains that a computed goto doesn't destroy objects that go out
of scope.  In the PR we agreed that it never will, but we can warn about
it.

	PR c++/37722

gcc/ChangeLog:

	* doc/extend.texi: Document that computed goto does not
	call destructors.

gcc/cp/ChangeLog:

	* decl.cc (identify_goto): Adjust for computed goto.
	(struct named_label_use_entry): Add computed_goto field.
	(poplevel_named_label_1): Add to computed_goto vec.
	(check_previous_goto_1): Dump computed_goto vec.
	(check_goto_1): Split out from check_goto.
	(check_goto): Check all addressable labels for computed goto.
	(struct named_label_entry): Add addressed field.
	(mark_label_addressed): New.
	* parser.cc (cp_parser_unary_expression): Call it.
	* cp-tree.h (mark_label_addressed): Declare it.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/label15.C: New test.
	* g++.dg/torture/pr42739.C: Expect warning.