[NoncopyableGenerics] remove incorrect fast-path

It's not technically correct to ask the generic signature without first
reducing the type, i.e., we really have to do this:

```
auto ty = sig->getReducedType(this)

// fast path
if (ty.isTypeParamter())
  return sig->requiresProtocol(s, copyable)

// slow path
mapTypeIntoContext(...)
```

Since reducing the type is basically what `mapTypeIntoContext` is also
going to do, it's not really worth doing this or having the fast-path at
 all. This need to reduce the type originally came while building the
 stdlib. We ran into a problem with this:

 ```
 extension Slice {
  public func initialize<S>(
    from source: S
  ) -> (unwritten: S.Iterator, index: Index)
    where S: Sequence, Base == UnsafeMutableBufferPointer<S.Element>
 }
 ```

 where we have `struct Slice<Base: Collection>` and

 ```
 extension Slice: Collection {
   typealias Index = Base.Index
   // ...
 }
 ```

Author: kavon

lib/AST/Type.cpp

@@ -181,20 +181,6 @@ static bool alwaysNoncopyable(Type ty) {
 /// context to substitute unbound types.
 bool TypeBase::isNoncopyable(const DeclContext *dc) {
   assert(dc);
-  auto &ctx = dc->getASTContext();
-
-  // Fast-path for type parameters; ask the generic signature directly.
-  if (isTypeParameter() &&
-      ctx.LangOpts.hasFeature(Feature::NoncopyableGenerics)) {
-    auto canType = getCanonicalType();
-
-    auto *copyable = ctx.getProtocol(KnownProtocolKind::Copyable);
-    if (!copyable)
-      llvm_unreachable("missing Copyable protocol!");
-
-    auto sig = dc->getGenericSignatureOfContext();
-    return !sig->requiresProtocol(canType, copyable);
-  }
 
   if (!hasArchetype() || hasOpenedExistential())
     if (auto env = dc->getGenericEnvironmentOfContext())