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

build/toolchain/win/BUILD.gn

Index: build/toolchain/win/BUILD.gn
diff --git a/build/toolchain/win/BUILD.gn b/build/toolchain/win/BUILD.gn
index 8efa40c26314fa1dfe14795242fe284f02863fee..dd1017fb3d9f4f08d0b2f6fa34e1d307b228956f 100644
--- a/build/toolchain/win/BUILD.gn
+++ b/build/toolchain/win/BUILD.gn
@@ -312,13 +312,6 @@ template("msvc_toolchain") {
   }
 }
 
-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 {
@@ -335,7 +328,6 @@ if (target_cpu == "x86") {
                                      windows_sdk_path,
                                      visual_studio_runtime_dirs,
                                      "x86",
-                                     "${sys_include_prefix}",
                                    ],
                                    "scope")
 
@@ -352,7 +344,7 @@ if (target_cpu == "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"
@@ -368,7 +360,6 @@ x64_toolchain_data = exec_script("setup_toolchain.py",
                                    windows_sdk_path,
                                    visual_studio_runtime_dirs,
                                    "x64",
-                                   "${sys_include_prefix}",
                                  ],
                                  "scope")
 
@@ -390,7 +381,7 @@ template("win_x64_toolchains") {
     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)) {