Fix instability in trailing clause body comments

[charliermarsh]

Summary

When we format the trailing comments on a clause body, we check if there are any newlines after the last statement; if not, we insert one.

This logic didn't take into account that the last statement could itself have trailing comments, as in:

if True:
    pass

    # comment
else:
    pass

We were thus inserting a newline after the comment, like:

if True:
    pass

    # comment

else:
    pass

In the context of function definitions, this led to an instability, since we insert a newline after a function, which would in turn lead to the bug above appearing in the second formatting pass.

Closes #7465.

Test Plan

cargo test

Small improvement in transformers, but no regressions.

Before:

project	similarity index	total files	changed files
cpython	0.76083	1789	1631
django	0.99983	2760	36
transformers	0.99956	2587	404
twine	1.00000	33	0
typeshed	0.99983	3496	18
warehouse	0.99967	648	15
zulip	0.99972	1437	21

After:

project	similarity index	total files	changed files
cpython	0.76083	1789	1631
django	0.99983	2760	36
transformers	0.99957	2587	402
twine	1.00000	33	0
typeshed	0.99983	3496	18
warehouse	0.99967	648	15
zulip	0.99972	1437	21