Provide structured suggestion for type mismatch in loop

We currently provide only a `help` message, this PR introduces the last
two structured suggestions instead:

```
error[E0308]: mismatched types
  --> $DIR/issue-98982.rs:2:5
   |
LL |   fn foo() -> i32 {
   |               --- expected `i32` because of return type
LL | /     for i in 0..0 {
LL | |         return i;
LL | |     }
   | |_____^ expected `i32`, found `()`
   |
note: the function expects a value to always be returned, but loops might run zero times
  --> $DIR/issue-98982.rs:2:5
   |
LL |     for i in 0..0 {
   |     ^^^^^^^^^^^^^ this might have zero elements to iterate on
LL |         return i;
   |         -------- if the loop doesn't execute, this value would never get returned
help: return a value for the case when the loop has zero elements to iterate on
   |
LL ~     }
LL ~     /* `i32` value */
   |
help: otherwhise consider changing the return type to account for that possibility
   |
LL ~ fn foo() -> Option<i32> {
LL |     for i in 0..0 {
LL ~         return Some(i);
LL ~     }
LL ~     None
   |
```

Fix rust-lang#98982.

Author: estebank

tests/ui/for-loop-while/break-while-condition.stderr

@@ -38,7 +38,7 @@ LL |             while false {
    |             ^^^^^^^^^^^ this might have zero elements to iterate on
 LL |                 return
    |                 ------ if the loop doesn't execute, this value would never get returned
-   = help: return a value for the case when the loop has zero elements to iterate on, or consider changing the return type to account for that possibility
+   = help: return a value for the case when the loop has zero elements to iterate on, otherwhise consider changing the return type to account for that possibility
 
 error: aborting due to 3 previous errors
 

tests/ui/typeck/issue-100285.rs

@@ -1,6 +1,5 @@
-fn foo(n: i32) -> i32 {
-    for i in 0..0 {
-    //~^ ERROR: mismatched types [E0308]
+fn foo(n: i32) -> i32 { //~ HELP otherwhise consider changing the return type to account for that possibility
+    for i in 0..0 { //~ ERROR mismatched types [E0308]
        if n < 0 {
         return i;
         } else if n < 10 {
@@ -15,8 +14,7 @@ fn foo(n: i32) -> i32 {
           return 5;
         }
 
-    }
-    //~| help: return a value for the case when the loop has zero elements to iterate on, or consider changing the return type to account for that possibility
+    } //~ HELP return a value for the case when the loop has zero elements to iterate on
 }
 
 fn main() {}

tests/ui/typeck/issue-100285.stderr

@@ -4,9 +4,9 @@ error[E0308]: mismatched types
 LL |   fn foo(n: i32) -> i32 {
    |                     --- expected `i32` because of return type
 LL | /     for i in 0..0 {
-LL | |
 LL | |        if n < 0 {
 LL | |         return i;
+LL | |         } else if n < 10 {
 ...  |
 LL | |
 LL | |     }
@@ -17,7 +17,7 @@ note: the function expects a value to always be returned, but loops might run ze
    |
 LL |     for i in 0..0 {
    |     ^^^^^^^^^^^^^ this might have zero elements to iterate on
-...
+LL |        if n < 0 {
 LL |         return i;
    |         -------- if the loop doesn't execute, this value would never get returned
 LL |         } else if n < 10 {
@@ -27,7 +27,32 @@ LL |         } else if n < 20 {
 LL |           return 2;
    |           -------- if the loop doesn't execute, this value would never get returned
    = note: if the loop doesn't execute, 3 other values would never get returned
-   = help: return a value for the case when the loop has zero elements to iterate on, or consider changing the return type to account for that possibility
+help: return a value for the case when the loop has zero elements to iterate on
+   |
+LL ~     }
+LL ~     /* `i32` value */
+   |
+help: otherwhise consider changing the return type to account for that possibility
+   |
+LL ~ fn foo(n: i32) -> Option<i32> {
+LL |     for i in 0..0 {
+LL |        if n < 0 {
+LL ~         return Some(i);
+LL |         } else if n < 10 {
+LL ~           return Some(1);
+LL |         } else if n < 20 {
+LL ~           return Some(2);
+LL |         } else if n < 30 {
+LL ~           return Some(3);
+LL |         } else if n < 40 {
+LL ~           return Some(4);
+LL |         } else {
+LL ~           return Some(5);
+LL |         }
+LL | 
+LL ~     }
+LL ~     None
+   |
 
 error: aborting due to previous error
 

tests/ui/typeck/issue-98982.rs

@@ -1,9 +1,7 @@
-fn foo() -> i32 {
-    for i in 0..0 {
-    //~^ ERROR: mismatched types [E0308]
+fn foo() -> i32 { //~ HELP otherwhise consider changing the return type to account for that possibility
+    for i in 0..0 { //~ ERROR mismatched types [E0308]
         return i;
-    }
-    //~| help: return a value for the case when the loop has zero elements to iterate on, or consider changing the return type to account for that possibility
+    } //~ HELP return a value for the case when the loop has zero elements to iterate on
 }
 
 fn main() {}

tests/ui/typeck/issue-98982.stderr

@@ -4,7 +4,6 @@ error[E0308]: mismatched types
 LL |   fn foo() -> i32 {
    |               --- expected `i32` because of return type
 LL | /     for i in 0..0 {
-LL | |
 LL | |         return i;
 LL | |     }
    | |_____^ expected `i32`, found `()`
@@ -14,10 +13,21 @@ note: the function expects a value to always be returned, but loops might run ze
    |
 LL |     for i in 0..0 {
    |     ^^^^^^^^^^^^^ this might have zero elements to iterate on
-LL |
 LL |         return i;
    |         -------- if the loop doesn't execute, this value would never get returned
-   = help: return a value for the case when the loop has zero elements to iterate on, or consider changing the return type to account for that possibility
+help: return a value for the case when the loop has zero elements to iterate on
+   |
+LL ~     }
+LL ~     /* `i32` value */
+   |
+help: otherwhise consider changing the return type to account for that possibility
+   |
+LL ~ fn foo() -> Option<i32> {
+LL |     for i in 0..0 {
+LL ~         return Some(i);
+LL ~     }
+LL ~     None
+   |
 
 error: aborting due to previous error