RequirementMachine: Overhaul handling of protocol typealiases with concrete underlying type

[slavapestov]

Consider this example:

protocol P {
  typealias A = Int
  associatedtype B
}

<T where T : P, T.A == T.B>

Recall that the concrete typealias introduces a rule of the form:

[P].A.[concrete: Int] => [P].A

This overlaps with the conformance rule (T.[P] => T) on the term T.[P].A to produce the rule T.A.[concrete: Int] => T.A.

Now the generic signature equates the associated type T.[P:B] with the typealias T.A via an abstract same-type requirement, which introduces a rewrite rule of the form:

T.A => T.[P:B]

This overlaps with T.A.[concrete: Int] => T.A on T.A.[concrete: Int], so we get a new rule:

T.[P:B].[concrete: Int] => T.[P:B]

For compatibility with the GenericSignatureBuilder, this final rule is the one we want to turn into a requirement in the minimal signature (T.B == Int); we don't want to keep (T.B == T.A) around.

To get this behavior, don't consider eliminating (T.[P:B].[concrete: Int] => T.[P:B]) via a rewrite loop containing any protocol typealias rules, in this case, ([P].A.[concrete: Int] => [P].A).

This trick only works because the left hand side of a protocol typelias rule is always greater than an associated type in the reduction order, since terms with name symbols are always greater than terms without name symbols.

There is a more general problem here where we fail to produce a signature compatible with the GenericSignatureBuilder in other similar cases that do not involve protocol typealiases, for example here we should transform U.Element == Character:

<T, U where T : Collection, U : Collection, T.SubSequence == Substring, T.Element == U.Element>

A solution to the above problem will subsume this hack, but for now, let's land it to move forward.

Fixes https://bugs.swift.org/browse/SR-15924 / rdar://problem/89641526.

[slavapestov]

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