fix(parser): avoid using Location::dummy()

[asterite]

Description

Problem

While working on another thing I noticed that LSP Document Symbol would crash on this code:

trait Bar {
    let N;
}

That is, an associated constant without a type. The reason it crashed is that the Document Symbol logic eventually relied on a dummy location.

Summary

Here all references to Location::dummy are replaced with a location right where the parser is currently is (an empty span at the last token end) which should prevent these crashes. I didn't add tests for these as they are many and in general locations aren't tested by the parser, but I verified the crash no longer happens.

Additional Context

Documentation

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[michaeljklein]

LGTM