Bug: Missing source map for variable inside optional?.chaining operator

Bug report

### Input Code

This shows the source maps the compiler returns for this line. Notice that the variable (`old_presences` in that case) is not being pointed at.

### Expected Behavior
Another source map from `old_presences` to `old_presences`

### Context
This is not problematic for normal users, but for IntelliSense from the [CoffeeSense LSP](https://github.com/phil294/coffeesense/) it would be necessary, as currently go-tos etc. are impossible around optional chaining vars.

That said, the source maps have been amazing otherwise. They are usually highly detailed and accurate so that I could successfully base the entire extension logic on them.

### Environment

* CoffeeScript version: 2.6.1
* Node.js version: 16.16.0

Start by reproducing the optional chaining source-map output with CoffeeScript 2.6.1 and inspect how the compiler maps the variable named old_presences. Compare the generated mappings with the expected old_presences-to-old_presences mapping; the issue is done when that mapping is emitted and the CoffeeSense go-to scenario can use it.

Written by the indexing model from the issue text.