Change return type of ExceptionalCondition to void and mark it noreturn

In ancient times, it was thought that this wouldn't work because of
TrapMacro/AssertMacro, but changing those to use a comma operator
appears to work without compiler warnings.

src/backend/utils/error/elog.c

@@ -1507,15 +1507,9 @@ pg_re_throw(void)
 		errfinish(0);
 	}
 
-	/* We mustn't return... */
+	/* Doesn't return ... */
 	ExceptionalCondition("pg_re_throw tried to return", "FailedAssertion",
 						 __FILE__, __LINE__);
-
-	/*
-	 * Since ExceptionalCondition isn't declared noreturn because of
-	 * TrapMacro(), we need this to keep gcc from complaining.
-	 */
-	abort();
 }