[SR-14285] Generic typealiases don't constrain their known types in extensions

[hartbit]

Previous ID	SR-14285
Radar	rdar://problem/74876586
Original Reporter	@hartbit
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: 45637aa4240ad1b88bdac3d67539c4b1

Issue Description:

I would expect the following code to compile and to define a secondUppercased property on all Tuple where the U type is a String.

struct Tuple<T, U> {
    let first: T
    let second: U
}

extension Tuple where U == String {
    var secondLowercased: String {
        second.lowercased()
    }
}

typealias TupleString<T> = Tuple<T, String>

extension TupleString {
    var secondUppercased: String {
        second.uppercased()
    }
}

[typesanitizer]

@swift-ci create