Fix formatter instability with half-indented comment (#6460)

## Summary
The bug was happening in this
[loop](75f402eb82/crates/ruff_python_formatter/src/comments/placement.rs (L545)).

Basically, In the first iteration of the loop, the `comment_indentation`
is bigger than `child_indentation` (`comment_indentation` is 7 and
`child_indentation` is 4) making the `Ordering::Greater` branch execute.
Inside the `Ordering::Greater` branch, the `if` block gets executed,
resulting in the update of these variables.
```rust
parent_body = current_body;                    
current_body = Some(last_child_in_current_body);
last_child_in_current_body = nested_child;
```
In the second iteration of the loop, `comment_indentation` is smaller
than `child_indentation` (`comment_indentation` is 7 and
`child_indentation` is 8) making the `Ordering::Less` branch execute.
Inside the `Ordering::Less` branch, the `if` block gets executed, this
is where the bug was happening. At this point `parent_body` should be a
`StmtFunctionDef` but it was a `StmtClassDef`. Causing the comment to be
incorrectly formatted.

That happened for the following code:
```python
class A:
    def f():
        pass
       # strangely indented comment

print()
```

There is only one problem that I couldn't figure it out a solution, the
variable `current_body` in this
[line](75f402eb82/crates/ruff_python_formatter/src/comments/placement.rs (L542C5-L542C49))
now gives this warning _"value assigned to `current_body` is never read
maybe it is overwritten before being read?"_
Any tips on how to solve that?

Closes #5337

## Test Plan

Add new test case.

---------

Co-authored-by: konstin <konstin@mailbox.org>

crates/ruff_python_formatter/tests/snapshots/format@statement__if.py.snap

@@ -109,6 +109,18 @@ else:
 if True: print("a") # 1
 elif True: print("b") # 2
 else: print("c") # 3
+
+# Regression test for https://github.com/astral-sh/ruff/issues/5337
+if parent_body:
+    if current_body:
+        child_in_body()
+        last_child_in_current_body()  # may or may not have children on its own
+# a
+  # b
+    # c
+      # d
+        # e
+          #f
 ```
 
 ## Output
@@ -224,6 +236,18 @@ elif True:
     print("b")  # 2
 else:
     print("c")  # 3
+
+# Regression test for https://github.com/astral-sh/ruff/issues/5337
+if parent_body:
+    if current_body:
+        child_in_body()
+        last_child_in_current_body()  # may or may not have children on its own
+        # e
+        # f
+    # c
+    # d
+# a
+# b
 ```