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
	* tree.c (block_may_fallthru): Return false if SWITCH_ALL_CASES_P
	is set on SWITCH_EXPR and !block_may_fallthru (SWITCH_BODY ()).
c/
	* c-typeck.c (c_finish_case): Set SWITCH_ALL_CASES_P if
	c_switch_covers_all_cases_p returns true.
c-family/
	* c-common.c (c_switch_covers_all_cases_p_1,
	c_switch_covers_all_cases_p): New functions.
	* c-common.h (c_switch_covers_all_cases_p): Declare.
testsuite/
	* c-c++-common/tsan/pr81275.c: New test.

From-SVN: r255217