[nullability] Allow nullability properties to be "top".

This is preparatory work for pointer widening; I'm submitting it for review
separately because:

* It separates this API change from the pointer widening itself.
* It shows that the API change is a no-op in terms of behavior (though note
  that, so far, we aren't covering the cases where the nullability properties
  are "top" -- this will come with the pointer widening).

PiperOrigin-RevId: 576854948
Change-Id: I3b13750193e682d831f930cb8fe030ec56b806a0
diff --git a/nullability/BUILD b/nullability/BUILD
index 3ba5bcf..b06dd32 100644
--- a/nullability/BUILD
+++ b/nullability/BUILD
@@ -117,6 +117,7 @@
     ],
     deps = [
         ":type_nullability",
+        "@absl//absl/base:nullability",
         "@llvm-project//clang:analysis",
         "@llvm-project//clang:ast",
         "@llvm-project//clang:basic",
@@ -133,6 +134,7 @@
         "@llvm-project//clang:analysis",
         "@llvm-project//clang:ast",
         "@llvm-project//clang:basic",
+        "@llvm-project//third-party/unittest:gmock",
         "@llvm-project//third-party/unittest:gtest",
         "@llvm-project//third-party/unittest:gtest_main",
     ],