Support custom Swift attributes

[tristanlabelle]

Related to #149 for newly authored classes.

    @MainActor
    public static func mainActor() throws {
        try _iswiftAttributesStatics.mainActor()
    }

    @available(swift 1)
    public static func availableFromSwift1() throws {
        try _iswiftAttributesStatics.availableFromSwift1()
    }

    @discardableResult
    @inlinable
    public static func inlinableWithDiscardableResult() throws -> Swift.Int32 {
        try _iswiftAttributesStatics.inlinableWithDiscardableResult()
    }