Fix closure coerced return type for add_return_type

Example
---
```rust
fn foo() {
    let f = ||$0 {loop {}};
    let _: fn() -> i8 = f;
}
```

**Before this PR**:

```rust
fn foo() {
    let f = || -> ! {loop {}};
    let _: fn() -> i8 = f;
}
```

mismatched types error on line 3

**After this PR**:

```rust
fn foo() {
    let f = || -> i8 {loop {}};
    let _: fn() -> i8 = f;
}
```

Author: A4-Tacks

src/tools/rust-analyzer/crates/ide-assists/src/handlers/add_return_type.rs

@@ -18,7 +18,7 @@ use crate::{AssistContext, AssistId, Assists};
 pub(crate) fn add_return_type(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<()> {
     let (fn_type, tail_expr, builder_edit_pos) = extract_tail(ctx)?;
     let module = ctx.sema.scope(tail_expr.syntax())?.module();
-    let ty = ctx.sema.type_of_expr(&peel_blocks(tail_expr.clone()))?.original();
+    let ty = ctx.sema.type_of_expr(&peel_blocks(tail_expr.clone()))?.adjusted();
     if ty.is_unit() {
         return None;
     }
@@ -421,6 +421,21 @@ mod tests {
         );
     }
 
+    #[test]
+    fn infer_coerced_return_type_closure() {
+        check_assist(
+            add_return_type,
+            r#"fn foo() {
+    let f = ||$0 {loop {}};
+    let _: fn() -> i8 = f;
+}"#,
+            r#"fn foo() {
+    let f = || -> i8 {loop {}};
+    let _: fn() -> i8 = f;
+}"#,
+        );
+    }
+
     #[test]
     fn not_applicable_ret_type_specified_closure() {
         cov_mark::check!(existing_ret_type_closure);