build: add a submodule for CX10 handling just the modules

The CX10 module setup is currently very complicated.  The module is
built twice - once via bazel and incorporated into the TensorFlow
library.  The second build is built via Swift for the modules to import
the interfaces into Swift.  In order to build the swift bindings via
SPM, we need to extract the sources.  However, we cannot reorganise the
CX10 subdirectory to enable SPM to filter out just the modules.  As a
result, we create a `CX10Modules` module that is only built when
building with SPM.  CMake builds will ignore this directory, building
the modules from the CX10 directory directly as it can handle the
subdirectory layout.

Package.swift

@@ -43,12 +43,16 @@ let package = Package(
     .target(
       name: "CTensorFlow",
       dependencies: []),
+    .target(
+      name: "CX10Modules",
+      dependencies: []),
     .target(
       name: "TensorFlow",
       dependencies: [
         "Tensor",
         "PythonKit",
         "CTensorFlow",
+        "CX10Modules",
         .product(name: "Numerics", package: "swift-numerics"),
       ],
       swiftSettings: [

Sources/CX10Modules/include/device_wrapper.h

@@ -0,0 +1 @@
+../../CX10/device_wrapper.h

Sources/CX10Modules/include/module.modulemap

@@ -0,0 +1 @@
+../../CX10/module.modulemap

Sources/CX10Modules/include/xla_tensor_tf_ops.h

@@ -0,0 +1 @@
+../../CX10/xla_tensor_tf_ops.h

Sources/CX10Modules/include/xla_tensor_wrapper.h

@@ -0,0 +1 @@
+../../CX10/xla_tensor_wrapper.h

Sources/CX10Modules/shim.c

@@ -0,0 +1,15 @@
+// Copyright 2019 The TensorFlow Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+extern int _;