Sema: Improve the 'protocol method can't impose new requirements on Self' check

[slavapestov]

This check had two problems. First, it would assert upon encountering a layout requirement, due to an unimplemented code path.

A more fundamental issue is that the logic wasn't fully sound, because it would miss certain cases, for example:

protocol P {
  associatedtype A

  func run<B: Equatable>(_: B) where B == Self.A
}

Here, the reduced type of Self.A is B, and at first glance, the requirement B: Equatable appears to be fine. However, this is actually a new requirement on Self, and the protocol be rejected.

Now that we can change the reduction order by assigning weights to generic parameters (#81171), this check can be implemented in a better way, by building a new generic signature first, where all generic parameters introduced by the protocol method, like 'B' above, are assigned a non-zero weight.

With this reduction order, any type that is equivalent to a member type of Self will have a reduced type rooted in Self, at which point the previous syntactic check becomes sound.

Since this may cause us to reject code we accepted previously, the type checker now performs the check twice: first on the original signature, which may miss certain cases, and then again on the new signature built with the weighted reduction order.

If the first check fails, we diagnose an error. If the second check fails, we only diagnose a warning.

However, I'd like to rip out the first check and turn the warning from the second check into an error soon. It really can cause compiler crashes and miscompiles to have a malformed protocol like this.

Fixes rdar://116938972.

[slavapestov]

@swift-ci Please smoke test