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

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 dfd1e84be24d52b5eb0ca60a554e9f02d9660db4..43b84b61ca52fa489c712067a73fa57cd648507e 100644
--- a/build/toolchain/win/BUILD.gn
+++ b/build/toolchain/win/BUILD.gn
@@ -55,7 +55,15 @@ template("msvc_toolchain") {
 
   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") {