Check exhaustiveness of function-ending match statements (#882)

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/testdata/tuple/match.carbon

@@ -17,4 +17,5 @@ fn main() -> i32 {
     case (a: auto, b: auto) =>
       return a + b - 7;
   }
+  return 1;
 }

executable_semantics/testdata/tuple/match_nested.carbon

@@ -19,4 +19,5 @@ fn main() -> i32 {
     case ((a: auto, b: auto), c: auto) =>
       return a - b + c[0] - c[1] + 2;
   }
+  return 1;
 }

executable_semantics/testdata/tuple/match_with_named.carbon

@@ -19,4 +19,5 @@ fn main() -> i32 {
     case (a: auto, .x = b: auto) =>
       return a - b + 3;
   }
+  return 1;
 }