[build-script] Add Product for libc++

In a non-legacy-impl world, this is needed to trigger
building/installing libc++.

https://bugs.swift.org/browse/SR-9861

Author: benlangmuir

utils/build-script

@@ -841,6 +841,7 @@ class BuildScriptInvocation(object):
         product_classes = []
         product_classes.append(products.CMark)
         product_classes.append(products.LLVM)
+        product_classes.append(products.LibCXX)
         if self.args.build_libicu:
             product_classes.append(products.LibICU)
         product_classes.append(products.Swift)

utils/swift_build_support/swift_build_support/products/__init__.py

@@ -13,6 +13,7 @@
 from .cmark import CMark
 from .foundation import Foundation
 from .libdispatch import LibDispatch
+from .libcxx import LibCXX
 from .libicu import LibICU
 from .llbuild import LLBuild
 from .lldb import LLDB
@@ -29,6 +30,7 @@
     'Ninja',
     'Foundation',
     'LibDispatch',
+    'LibCXX',
     'LibICU',
     'LLBuild',
     'LLDB',

utils/swift_build_support/swift_build_support/products/libcxx.py

@@ -0,0 +1,17 @@
+# swift_build_support/products/libcxx.py -------------------------*- python -*-
+#
+# This source file is part of the Swift.org open source project
+#
+# Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+# Licensed under Apache License v2.0 with Runtime Library Exception
+#
+# See https://swift.org/LICENSE.txt for license information
+# See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+#
+# ----------------------------------------------------------------------------
+
+from . import product
+
+
+class LibCXX(product.Product):
+    pass