re PR sanitizer/81275 (-fsanitize=thread produce incorrect -Wreturn-type warning)

re PR sanitizer/81275 (-fsanitize=thread produce incorrect -Wreturn-type warning)

	PR sanitizer/81275
	* cp-tree.h (SWITCH_STMT_ALL_CASES_P): Define.
	(SWITCH_STMT_NO_BREAK_P): Define.
	(note_break_stmt, note_iteration_stmt_body_start,
	note_iteration_stmt_body_end): Declare.
	* decl.c (struct cp_switch): Add has_default_p, break_stmt_seen_p
	and in_loop_body_p fields.
	(push_switch): Clear them.
	(pop_switch): Set SWITCH_STMT_CANNOT_FALLTHRU_P if has_default_p
	and !break_stmt_seen_p.  Assert in_loop_body_p is false.
	(note_break_stmt, note_iteration_stmt_body_start,
	note_iteration_stmt_body_end): New functions.
	(finish_case_label): Set has_default_p when both low and high
	are NULL_TREE.
	* parser.c (cp_parser_iteration_statement): Use
	note_iteration_stmt_body_start and note_iteration_stmt_body_end
	around parsing iteration body.
	* pt.c (tsubst_expr): Likewise.
	* cp-objcp-common.c (cxx_block_may_fallthru): Return false for
	SWITCH_STMT which contains no BREAK_STMTs, contains a default:
	CASE_LABEL_EXPR and where SWITCH_STMT_BODY isn't empty and
	can't fallthru.
	* semantics.c (finish_break_stmt): Call note_break_stmt.
	* cp-gimplify.c (genericize_switch_stmt): Copy SWITCH_STMT_ALL_CASES_P
	bit to SWITCH_ALL_CASES_P.  Assert that if SWITCH_STMT_NO_BREAK_P then
	the break label is not TREE_USED.

	* g++.dg/warn/pr81275-1.C: New test.
	* g++.dg/warn/pr81275-2.C: New test.
	* g++.dg/warn/pr81275-3.C: New test.
	* c-c++-common/tsan/pr81275.c: Skip for C++ and -O2.

From-SVN: r255218