PathFragment: test Windows mixed separator

On Windows, if a user were to specify a PathFragment with mixed
separator such as

```
bazel build --profile='C:/foo\\bar' //...
```

then the profile file will be written as `bar` and recorded in
BuildToolLogs event as so.

Provide a test that demonstrates the current behavior to discuss whether
this is "correct" or not.

src/test/java/com/google/devtools/build/lib/vfs/PathFragmentWindowsTest.java

@@ -193,6 +193,8 @@ public void testBasenameWindows() throws Exception {
     assertThat(create("C:/foo").getBaseName()).isEqualTo("foo");
     // Never return the drive name as a basename.
     assertThat(create("C:/").getBaseName()).isEmpty();
+    // Mix usage of Windows and Unix separator is valid
+    assertThat(create("C:/foo\\bar").getBaseName()).isEqualTo("bar");
   }
 
   @Test