use clang on Windows (flutter/engine#17407)

* use clang on Windows for everything but Android

This pulls the Clang build from Chromium. We should eventually switch to use the Fuchsia toolchain when it is available.

DEPS

@@ -527,6 +527,18 @@ deps = {
     'dep_type': 'cipd',
   },
 
+  # TODO(fxb/4443): Remove this when Fuchsia can provide the Windows Clang Toolchain
+  'src/third_party/llvm-build/Release+Asserts': {
+    'packages': [
+      {
+        'package': 'flutter/clang/win-amd64',
+        'version': 'git_revision:5ec206df8534d2dd8cb9217c3180e5ddba587393'
+      }
+    ],
+    'condition': 'download_windows_deps',
+    'dep_type': 'cipd',
+  },
+
    # Get the SDK from https://chrome-infra-packages.appspot.com/p/fuchsia/sdk/core at the 'latest' tag
    # Get the toolchain from https://chrome-infra-packages.appspot.com/p/fuchsia/clang at the 'goma' tag
 
@@ -624,6 +636,15 @@ hooks = [
       'src/third_party/dart/third_party/7zip.tar.gz.sha1',
     ],
   },
+  {
+    'name': 'dia_dll',
+    'pattern': '.',
+    'condition': 'download_windows_deps',
+    'action': [
+      'python',
+      'src/flutter/tools/dia_dll.py',
+    ],
+  },
   {
     'name': 'linux_sysroot',
     'pattern': '.',

engine/src/flutter/tools/dia_dll.py

@@ -0,0 +1,75 @@
+#!/usr/bin/env python
+# 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.
+
+"""
+This script is based on chromium/chromium/master/tools/clang/scripts/update.py.
+
+It is used on Windows platforms to copy the correct msdia*.dll to the
+clang folder, as a "gclient hook".
+"""
+
+import os
+import shutil
+import stat
+import sys
+
+
+# Path constants. (All of these should be absolute paths.)
+THIS_DIR = os.path.abspath(os.path.dirname(__file__))
+LLVM_BUILD_DIR = os.path.abspath(os.path.join(THIS_DIR, '..', '..', 'third_party',
+                                              'llvm-build', 'Release+Asserts'))
+
+
+def GetDiaDll():
+  """Get the location of msdia*.dll for the platform."""
+
+  # Bump after VC updates.
+  DIA_DLL = {
+    '2013': 'msdia120.dll',
+    '2015': 'msdia140.dll',
+    '2017': 'msdia140.dll',
+    '2019': 'msdia140.dll',
+  }
+
+  # Don't let vs_toolchain overwrite our environment.
+  environ_bak = os.environ
+
+  sys.path.append(os.path.join(THIS_DIR, '..', '..', 'build'))
+  import vs_toolchain
+  win_sdk_dir = vs_toolchain.SetEnvironmentAndGetSDKDir()
+  msvs_version = vs_toolchain.GetVisualStudioVersion()
+
+  if bool(int(os.environ.get('DEPOT_TOOLS_WIN_TOOLCHAIN', '1'))):
+    dia_path = os.path.join(win_sdk_dir, '..', 'DIA SDK', 'bin', 'amd64')
+  else:
+    if 'GYP_MSVS_OVERRIDE_PATH' in os.environ:
+      vs_path = os.environ['GYP_MSVS_OVERRIDE_PATH']
+    else:
+      vs_path = vs_toolchain.DetectVisualStudioPath()
+    dia_path = os.path.join(vs_path, 'DIA SDK', 'bin', 'amd64')
+
+  os.environ = environ_bak
+  return os.path.join(dia_path, DIA_DLL[msvs_version])
+
+
+def CopyFile(src, dst):
+  """Copy a file from src to dst."""
+  print("Copying %s to %s" % (str(src), str(dst)))
+  shutil.copy(src, dst)
+
+
+def CopyDiaDllTo(target_dir):
+  # This script always wants to use the 64-bit msdia*.dll.
+  dia_dll = GetDiaDll()
+  CopyFile(dia_dll, target_dir)
+
+
+def main():
+  CopyDiaDllTo(os.path.join(LLVM_BUILD_DIR, 'bin'))
+  return 0
+
+
+if __name__ == '__main__':
+  sys.exit(main())

engine/src/flutter/tools/gn

@@ -101,7 +101,13 @@ def to_gn_args(args):
     gn_args['dart_component_kind'] = 'static_library' # Always link Dart in statically.
     gn_args['is_debug'] = args.unoptimized
     gn_args['android_full_debug'] = args.target_os == 'android' and args.unoptimized
-    gn_args['is_clang'] = not sys.platform.startswith(('cygwin', 'win'))
+    if args.clang is None:
+      # Android gen_snapshot currently isn't buildable with clang on Windows.
+      # For everything else, default to clang.
+      gn_args['is_clang'] = not (sys.platform.startswith(('cygwin', 'win')) and
+                                 args.target_os == 'android')
+    else:
+      gn_args['is_clang'] = args.clang
 
     if args.target_os == 'android' or args.target_os == 'ios':
       gn_args['skia_gl_standard'] = 'gles'
@@ -314,8 +320,8 @@ def parse_args(args):
   parser.add_argument('--lto', default=True, action='store_true')
   parser.add_argument('--no-lto', dest='lto', action='store_false')
 
-  parser.add_argument('--clang', default=True, action='store_true')
-  parser.add_argument('--no-clang', dest='clang', action='store_false')
+  parser.add_argument('--clang', action='store_const', const=True)
+  parser.add_argument('--no-clang', dest='clang', action='store_const', const=False)
 
   parser.add_argument('--clang-static-analyzer', default=False, action='store_true')
   parser.add_argument('--no-clang-static-analyzer', dest='clang_static_analyzer', action='store_false')