clang/win: Start work on getting clang/win working in gn.

build/common.gypi

 # Copyright (c) 2012 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 5683 matching lines @@
               'VCManifestTool': {
                 'AdditionalManifestFiles': [
                   '>(win_exe_compatibility_manifest)',
                 ],
               },
             }],
           ],
           'conditions': [
             # Building with Clang on Windows is a work in progress and very
             # experimental. See crbug.com/82385.
+            # Keep this in sync with the similar blocks in build/config/compiler/BUILD.gn
             ['clang==1', {
               'VCCLCompilerTool': {
                 'AdditionalOptions': [
                   '-fmsc-version=1800',
 
                   # Many files use intrinsics without including this header.
                   # TODO(hans): Fix those files, or move this to sub-GYPs.
                   '/FIIntrin.h',
 
                   # TODO(hans): Make this list shorter eventually.
@@ skipping 474 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }