Suggest turnging if let into irrefutable let if appropriate

When encountering an `if let` tail expression without an `else` arm for an
enum with a single variant, suggest writing an irrefutable `let` binding
instead.

```
error[E0317]: `if` may be missing an `else` clause
  --> $DIR/irrefutable-if-let-without-else.rs:8:5
   |
LL |   fn foo(x: Enum) -> i32 {
   |                      --- expected `i32` because of this return type
LL | /     if let Enum::Variant(value) = x {
LL | |         value
LL | |     }
   | |_____^ expected `i32`, found `()`
   |
   = note: `if` expressions without `else` evaluate to `()`
   = help: consider adding an `else` block that evaluates to the expected type
help: consider using an irrefutable `let` binding instead
   |
LL ~     let Enum::Variant(value) = x;
LL ~         value
   |
```

Fix #61788.

tests/ui/binding/irrefutable-if-let-without-else.fixed

@@ -0,0 +1,25 @@
+// run-rustfix
+enum Enum {
+    Variant(i32),
+}
+struct Struct(i32);
+
+fn foo(x: Enum) -> i32 {
+    let Enum::Variant(value) = x;
+        value
+}
+fn bar(x: Enum) -> i32 {
+    let Enum::Variant(value) = x;
+        let x = value + 1;
+        x
+}
+fn baz(x: Struct) -> i32 {
+    let Struct(value) = x;
+        let x = value + 1;
+        x
+}
+fn main() {
+    let _ = foo(Enum::Variant(42));
+    let _ = bar(Enum::Variant(42));
+    let _ = baz(Struct(42));
+}