Issue Description:

In the Swift generated interface, extensions to generic types with constraints on their generic parameters all show that constraint. This implies that the extensions (and the conformances they add) are conditionally available, even though they're always there. For example, Set constrains its Element type to Hashable, and every extension for Set in the generated interface shows the where Element : Hashable constraint:

extension Set : ExpressibleByArrayLiteral where Element : Hashable {

    /// Creates a set containing the elements of the given array literal.
    /// ...
    public init(arrayLiteral elements: Set<Element>.Element...)
}

extension Set : Sequence where Element : Hashable {

    /// Returns an iterator over the members of the set.
    public func makeIterator() -> SetIterator<Element>

    /// Returns a Boolean value that indicates whether the given element exists
    /// in the set.
    /// ...
    public func contains(_ member: Set<Element>.Element) -> Bool
}

This is technically correct, since the constraint is met for all these extensions, but it's superfluous and confusing for readers of the interface.