clang/win: Start work on getting clang/win working in gn.

build/config/BUILDCONFIG.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 if (target_os == "") {
   target_os = host_os
 }
 
 if (target_cpu == "") {
   if (target_os == "android") {
@@ skipping 506 matching lines @@
 # TOOLCHAIN SETUP
 # ==============================================================================
 #
 # Here we set the default toolchain, as well as the variable host_toolchain
 # which will identify the toolchain corresponding to the local system when
 # doing cross-compiles. When not cross-compiling, this will be the same as the
 # default toolchain.
 
 if (is_win) {
   # On windows we use the same toolchain for host and target by default.
-  # TODO(dpranke): rename the toolchains to x64 and x86 to match current_cpu.
-  if (current_cpu == "x64") {
-    host_toolchain = "//build/toolchain/win:64"
-  } else if (current_cpu == "x86") {
-    host_toolchain = "//build/toolchain/win:32"
+  if (is_clang) {
+    host_toolchain = "//build/toolchain/win:clang_$current_cpu"
+  } else {
+    host_toolchain = "//build/toolchain/win:$current_cpu"
   }
   set_default_toolchain("$host_toolchain")
 } else if (is_android) {
   if (host_os == "linux") {
     # Use clang for the x86/64 Linux host builds.
     if (host_cpu == "x86" || host_cpu == "x64") {
       host_toolchain = "//build/toolchain/linux:clang_$host_cpu"
     } else {
       host_toolchain = "//build/toolchain/linux:$host_cpu"
     }
@@ skipping 228 matching lines @@
       }
       if (defined(invoker.testonly)) {
         testonly = invoker.testonly
       }
       if (defined(invoker.visibility)) {
         visibility = invoker.visibility
       }
     }
   }
 }