Insert necessary blank line between class and leading comments

[charliermarsh]

Summary

Given:

# comment

class A:
    def foo(self):
        pass

We need to insert an additional newline between # comment and class A. We were missing this handling for the case in which # comment is a leading comment on class A, as opposed to a trailing comment of some preceding statement.

In practice, I think this only applies to the specific case in which a class or function is the first statement in a module, and there's a single empty line between a leading comment and that class or function. If there are no empty lines, then the comment "sticks" to the definition; if there are two or more, then leading_comments will truncate appropriately. If the class or function is nested, then we only need one empty line anyway.

Closes #8215.

Test Plan

No change in similarity.

Before:

project	similarity index	total files	changed files
cpython	0.75803	1799	1647
django	0.99983	2772	34
home-assistant	0.99953	10596	186
poetry	0.99891	317	17
transformers	0.99966	2657	330
twine	1.00000	33	0
typeshed	0.99978	3669	20
warehouse	0.99977	654	13
zulip	0.99970	1459	22

After:

project	similarity index	total files	changed files
cpython	0.75803	1799	1648
django	0.99983	2772	34
home-assistant	0.99953	10596	186
poetry	0.99891	317	17
transformers	0.99966	2657	330
twine	1.00000	33	0
typeshed	0.99978	3669	20
warehouse	0.99977	654	13
zulip	0.99970	1459	22