Added function signature to expand diff line

What does this MR do and why?

Shows the function signature in diffs below the expand diffs button.

Screenshots or screen recordings

Inline	Parallel

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Closes #20116 (closed)