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/assert.c

@@ -21,11 +21,8 @@
 
 /*
  * ExceptionalCondition - Handles the failure of an Assert()
- *
- * Note: this can't actually return, but we declare it as returning int
- * because the TrapMacro() macro might get wonky otherwise.
  */
-int
+void
 ExceptionalCondition(const char *conditionName,
 					 const char *errorType,
 					 const char *fileName,
@@ -55,6 +52,4 @@ ExceptionalCondition(const char *conditionName,
 #endif
 
 	abort();
-
-	return 0;
 }