Related plugin:
https://marketplace.atlassian.com/plugins/com.madgnome.stash.plugins.buccaneer

However, the plugin:
1. doesn't work (seems abandoned)
2. is designed only for Source view, not for Pull Request Diff view

This would greatly improve code review: If I'm inspecting a piece of code and encounter a method call of a method which implementation I wish to inspect, I just click on the method name in the method call and I'm taken to the method implementation (or a list of locations where the method might be is suggested).
Analogous behavior for types (classes or whatever).

Also: If the location is in the Diff list, the link lead out of the Diff view.