Fix private properties not being allowed in optional chains

[Psychpsyo]

Fixes #42734

This makes it so that expressions of the form foo?.#bar or foo?.bar.#baz are no longer seen as invalid, since the relevant JavaScript standards allow these by now and TypeScript claims to be a superset of JavaScript.
Specifically, private properties are now allowed inside an optional chain.
This syntax is allowed since tc39/proposal-class-fields#301

The added this.a = this; in privateIdentifierChain.1.ts is there so that line 14, this?.a.#b;, does not error because a might be undefined, as that's not what this file is testing for.

I'm sorry for fixing an issue that is not in the backlog, but this is bothering me personally and has existed for years at this point.
Otherwise, I have verified that:

• Code is up-to-date with the main branch
• You've successfully run hereby runtests locally
• There are new or updated unit tests validating the change

[Psychpsyo]

@microsoft-github-policy-service agree