win: Only specify /X flag to rc for clang.

This fixes the MSVC build in ANGLE.

Bug: 961769
Bug: 1017514
Change-Id: I40b5590bb700dd2d729a15a8088b97c13ebb1df5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879319
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709374}

build/toolchain/win/rc/rc.py

@@ -123,7 +123,9 @@ def Preprocess(rc_file_data, flags):
   # Closing temp_handle immediately defeats the purpose of mkstemp(), but I
   # can't figure out how to let write to the temp file on Windows otherwise.
   os.close(temp_handle)
-  clang_cmd = [clang, '/P', '/X', '/DRC_INVOKED', '/TC', '-', '/Fi' + temp_file]
+  clang_cmd = [clang, '/P', '/DRC_INVOKED', '/TC', '-', '/Fi' + temp_file]
+  if flags.imsvcs:
+    clang_cmd += ['/X']
   if os.path.dirname(flags.input):
     # This must precede flags.includes.
     clang_cmd.append('-I' + os.path.dirname(flags.input))