[Sema] Allow self-assignment with parenthesized right-side

[aenewton]

Per the discussion in the Jira ticket, this PR restores the ability to perform self-assignments if the right side of the expression is wrapped in parentheses:
self.name = (self.name)

Resolves SR-4464.

[slavapestov]

This needs a test, since it is a functional change.

[slavapestov]

If the right hand side is not an lvalue (e.g., you are assigning the result of a function call) this returns null and you will hit a null pointer deref on the next line. You should unwrap the LoadExpr if one is there, but otherwise keep going.

[aenewton]

Thanks for the feedback, @slavapestov. I'll change this and make a test

[CodaFi]

dyn_casts with unused rvalues should be isas.

[CodaFi]

It would also be great to leave some comments here explaining the behavior change. It's not obvious why a parenthesized rvalue gets to invoke side effects and a regular assignment doesn't.

[aenewton]

Thanks, @CodaFi. I'm actually unsure of the reason behind the change. I picked up the ticket because it seemed like a good place to start. It seems to me like the parenthesis would be allowed because it's an explicit indication that you're doing it for the side-effects, but maybe @jrose-apple has a better explanation.

[slavapestov]

My understanding is that the parens are an "escape hatch" - we need some way to express self-assignment in the case where it's really what you want.