[Sema] fix tuples and NoncopyableGenerics

The generic signature for the built-in TupleDecl was incorrect, stating
that the Element type must be Copyable. That in combination with an
extension that says tuples are Copyable when Element is copyable was
giving unconditional Copyable conformances for tuples that never
actually get checked.

Given that the existence of a conformance and no out-standing
conditional requirements is now what is used to determine if a value is
noncopyable, this was causing tuples with noncopyable elements to report
 that they are copyable in `isNoncopyable` requests.

Author: kavon

lib/AST/ASTContext.cpp

@@ -6174,6 +6174,17 @@ BuiltinTupleDecl *ASTContext::getBuiltinTupleDecl() {
   result = new (*this) BuiltinTupleDecl(Id_TheTupleType, dc);
   result->setAccess(AccessLevel::Public);
 
+  // Avoid going through InferredGenericSignatureRequest and directly set the
+  // generic signature to <each Element>
+  {
+    GenericParamList *list = result->getGenericParams();
+    assert(list->size() == 1);
+    auto paramTy = (*list->begin())->getDeclaredInterfaceType()
+                                   ->castTo<GenericTypeParamType>();
+    auto baseSig = GenericSignature::get({paramTy}, {});
+    result->setGenericSignature(baseSig);
+  }
+
   // Cook up conditional conformances to Sendable and Copyable.
   auto buildFakeExtension = [&](ProtocolDecl *proto) {
     auto protoTy = proto->getDeclaredInterfaceType();