Tell clang-cl to use POSIX quoting for response files when running on non-Windows hosts.

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/sanitizers/sanitizers.gni")
 import("//build/config/win/visual_studio_version.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/toolchain.gni")
 
 # Should only be running on Windows.
@@ skipping 37 matching lines @@
 #  toolchain_cpu: current_cpu to pass as a build arg
 #  toolchain_os: current_os to pass as a build arg
 #  environment: File name of environment file.
 template("msvc_toolchain") {
   if (defined(invoker.concurrent_links)) {
     concurrent_links = invoker.concurrent_links
   }
 
   env = invoker.environment
 
-  cl = invoker.cl
+  if (invoker.is_clang && host_os != "win") {
+    # This toolchain definition uses response files for compilations.  GN uses
+    # the quoting rules of the host OS, while clang-cl always defaults to
+    # cmd.exe quoting rules for parsing response files.  Tell clang-cl to use
+    # POSIX quoting rules, so it can understand what GN generates.
+    cl = "${invoker.cl} --rsp-quoting=posix"
+  } else {
+    cl = invoker.cl
+  }
 
   if (use_lld) {
     if (host_os == "win") {
       lld_link = "lld-link.exe"
     } else {
       lld_link = "lld-link"
     }
     prefix = rebase_path("//third_party/llvm-build/Release+Asserts/bin",
                          root_build_dir)
 
@@ skipping 314 matching lines @@
 
   msvc_toolchain("winrt_x64") {
     environment = "environment.winrt_x64"
     cl = "${goma_prefix}\"${vc_bin_dir}/cl.exe\""
     is_clang = false
 
     toolchain_cpu = "x64"
     toolchain_os = current_os
   }
 }