Use -fdiagnostics-absolute-paths in Win-Clang builds

build/config/compiler/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/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/toolchain.gni")
@@ skipping 354 matching lines @@
   if (linux_use_bundled_binutils) {
     cflags += [ "-B$binutils_path" ]
   }
 
   # Clang-specific compiler flags setup.
   # ------------------------------------
   if (is_clang) {
     cflags += [ "-fcolor-diagnostics" ]
   }
 
+  # Print absolute paths in diagnostics. There is no precedent for doing this
+  # on Linux/Mac (GCC doesn't support it), but MSVC does this with /FC and
+  # Windows developers rely on it (crbug.com/636109) so only do this on Windows.
+  # TODO(hans) Remove llvm_force_head_revision check in next clang roll.
+  if (is_clang && is_win && llvm_force_head_revision) {
+    cflags += [ "-fdiagnostics-absolute-paths" ]
+  }
+
   # Makes builds independent of absolute file path.
   # clang-cl (used if is_win) doesn't expose this flag.
   # Currently disabled for nacl since its toolchain lacks this flag (too old).
   # TODO(zforman): Once nacl's toolchain is updated, remove check.
   if (is_clang && is_linux) {
     absolute_path = rebase_path("//.")
     cflags += [ "-fdebug-prefix-map=$absolute_path=." ]
   }
 
   # C++11 compiler flags setup.
@@ skipping 1199 matching lines @@
 
 if (is_ios || is_mac) {
   # On Mac and iOS, this enables support for ARC (automatic ref-counting).
   # See http://clang.llvm.org/docs/AutomaticReferenceCounting.html.
   config("enable_arc") {
     common_flags = [ "-fobjc-arc" ]
     cflags_objc = common_flags
     cflags_objcc = common_flags
   }
 }