Always `==` not `=` in `where` clauses

[josh11b]

Instead of having two different operators and semantics when constraining an associated type to be equal to another type variable versus a concrete type, unify the semantics and consistently use just the == operator in where clauses.

[jonmeow]

Seems like a good change to me. I might remark = feels like assignment, but where feels more like a condition. It might particularly be worth noting that in examples like let PointT:! NSpacePoint where .N = 2;, let supports assignment following so using == to make it read less like an assignment might improve readability.

[josh11b]

It might particularly be worth noting that in examples like let PointT:! NSpacePoint where .N = 2;, let supports assignment following so using == to make it read less like an assignment might improve readability.

I added this rationale to the alternatives considered: https://github.com/carbon-language/carbon-lang/pull/2070/commits/fefcce63c94cc6cbd52ceb58e7f75bf5c9f21bcc

[josh11b]

I'm going to withdraw this proposal in favor of #2173 .