win: Decide how to include system dirs at toolchain time instead of globally.

build/toolchain/win/BUILD.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.
 
 import("//build/config/clang/clang.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/win/visual_studio_version.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/toolchain.gni")
@@ skipping 294 matching lines @@
       command = stamp_command
       description = stamp_description
     }
     tool("copy") {
       command = copy_command
       description = copy_description
     }
   }
 }
 
-if (is_clang) {
-  sys_include_prefix = "-imsvc"
-} else {
-  # MSVC doesn't have the concept of system headers.
-  sys_include_prefix = "/I"
-}
-
 if (host_os == "win") {
   clang_cl = "clang-cl.exe"
 } else {
   clang_cl = "clang-cl"
 }
 
 # 32-bit toolchains. Only define these when the target architecture is 32-bit
 # since we don't do any 32-bit cross compiles when targeting 64-bit (the
 # build does generate some 64-bit stuff from 32-bit target builds).
 if (target_cpu == "x86") {
   x86_toolchain_data = exec_script("setup_toolchain.py",
                                    [
                                      visual_studio_path,
                                      windows_sdk_path,
                                      visual_studio_runtime_dirs,
                                      "x86",
-                                     "${sys_include_prefix}",
                                    ],
                                    "scope")
 
   msvc_toolchain("x86") {
     environment = "environment.x86"
     cl = "${goma_prefix}\"${x86_toolchain_data.vc_bin_dir}/cl.exe\""
     toolchain_args = {
       current_cpu = "x86"
       is_clang = false
     }
   }
 
   msvc_toolchain("clang_x86") {
     environment = "environment.x86"
     prefix = rebase_path("$clang_base_path/bin", root_build_dir)
     cl = "${goma_prefix}$prefix/${clang_cl}"
-    sys_include_flags = "${x86_toolchain_data.include_flags}"
+    sys_include_flags = "${x86_toolchain_data.include_flags_imsvc}"
 
     toolchain_args = {
       current_cpu = "x86"
       is_clang = true
     }
   }
 }
 
 # 64-bit toolchains.
 x64_toolchain_data = exec_script("setup_toolchain.py",
                                  [
                                    visual_studio_path,
                                    windows_sdk_path,
                                    visual_studio_runtime_dirs,
                                    "x64",
-                                   "${sys_include_prefix}",
                                  ],
                                  "scope")
 
 template("win_x64_toolchains") {
   msvc_toolchain(target_name) {
     environment = "environment.x64"
     cl = "${goma_prefix}\"${x64_toolchain_data.vc_bin_dir}/cl.exe\""
 
     toolchain_args = {
       if (defined(invoker.toolchain_args)) {
         forward_variables_from(invoker.toolchain_args, "*")
       }
       is_clang = false
       current_cpu = "x64"
     }
   }
 
   msvc_toolchain("clang_" + target_name) {
     environment = "environment.x64"
     prefix = rebase_path("$clang_base_path/bin", root_build_dir)
     cl = "${goma_prefix}$prefix/${clang_cl}"
-    sys_include_flags = "${x64_toolchain_data.include_flags}"
+    sys_include_flags = "${x64_toolchain_data.include_flags_imsvc}"
 
     toolchain_args = {
       if (defined(invoker.toolchain_args)) {
         forward_variables_from(invoker.toolchain_args, "*")
       }
       is_clang = true
       current_cpu = "x64"
     }
   }
 }
@@ skipping 35 matching lines @@
   msvc_toolchain("winrt_x64") {
     environment = "environment.winrt_x64"
     cl = "${goma_prefix}\"${vc_bin_dir}/cl.exe\""
 
     toolchain_args = {
       is_clang = false
       current_cpu = "x64"
     }
   }
 }