message: [unittest] Fix the FileSystem test on Windows.
  On Windows, when using the VFS without going through FileSpec, the
  absolute path to `/foo` is `\\foo`. This updates the unittest to expect
  that.
diff: @@ -225,7 +225,11 @@ TEST(FileSystemTest, MakeAbsolute) {
  SmallString<16> foo(foo_relative);
  auto EC = fs.MakeAbsolute(foo);
  EXPECT_FALSE(EC);
  +#ifdef _WIN32
  + EXPECT_TRUE(foo.equals("\\\\foo"));
  +#else
  EXPECT_TRUE(foo.equals("/foo"));
  +#endif
  }
  {
  @@ -243,7 +247,11 @@ TEST(FileSystemTest, Resolve) {
  StringRef foo_relative = "foo";
  SmallString<16> foo(foo_relative);
  fs.Resolve(foo);
  +#ifdef _WIN32
  + EXPECT_TRUE(foo.equals("\\\\foo"));
  +#else
  EXPECT_TRUE(foo.equals("/foo"));
  +#endif
  }
  {
  