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

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index aa22d90bfdec0c78ae06ef1d3b7b62a41e75751e..d0daf4f3d7ccfb5853a859ea9d21e5620b938f2d 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -372,6 +372,14 @@ config("compiler") {
     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).