extend.texi (Attribute Syntax): Document that C++ labels on empty statements can now have attributes.

./:	* doc/extend.texi (Attribute Syntax): Document that C++ labels on
	empty statements can now have attributes.
cp/:
	* parser.c (cp_parser_label_for_labeled_statement): Support
	attribute on labels if immediately followed by semicolon.
	* semantics.c (finish_label_stmt): Return new label.
	* pt.c (tsubst_expr): Handle attributes for LABEL_EXPR.
testsuite/:
	* gcc.dg/Wunused-label-1.c: New test case.
	* g++.dg/warn/Wunused-label-1.C: New test case.
	* g++.dg/warn/Wunused-label-2.C: New test case.
	* g++.dg/warn/Wunused-label-3.C: New test case.

From-SVN: r148242