[SR-8239][Parse] Fix same-type constraint commutativity

[AnthonyLatsis]

Same-type constraints are not always commutative, i.e. (see SR for more)

protocol P { associatedtype Assoc }
func foo<U, T: P>(...) where T.Assoc == U? { }  // ok

func foo<U, T: P>(...) where U? == T.Assoc { }  // 4 parsing errors

Resolves SR-8239.

[AnthonyLatsis]

cc @slavapestov

[rintaro]

Could you add test case for : requirement? For example where [T]: Equatable, where T & Collection: Sequence

[slavapestov]

Looks good! As @rintaro mentioned it would be good to ensure that invalid cases that can now parsed are correctly rejected without crashing.

[slavapestov]

@swift-ci Please smoke test

[AnthonyLatsis]

I think it makes sense to leave the error message as "expected type".

[slavapestov]

@swift-ci Please smoke test

[AnthonyLatsis]

Thank you for reviewing!