NCGenerics: restrict conditional Copyable reqs

It doesn't really make sense for a conditional conformance requirement
for `Copyable` to depend on any other requirement other than other
`Copyable` conformance requirements.

resolves rdar://124967739

test/ModuleInterface/Inputs/NoncopyableGenerics_Misc.swift

@@ -98,7 +98,7 @@ extension Outer: Copyable {}
 extension Outer.InnerStruct: Copyable {}
 
 extension Outer.InnerVariation1: Copyable {}
-extension Outer.InnerVariation2: Escapable {}
+extension Outer.InnerVariation2: Escapable where A: ~Copyable {}
 
 extension Outer.InnerStruct {
     public func hello<T: ~Escapable>(_ t: T) {}