Improve nullability check and annoations for nbest_generator.

This CL improves the nullability check by using:
  * absl::Nonnull<> and absl::Nullable<>
  * Reference whenever possible

PiperOrigin-RevId: 648654106

src/base/container/BUILD.bazel

@@ -57,7 +57,10 @@ mozc_cc_library(
     name = "freelist",
     hdrs = ["freelist.h"],
     visibility = ["//:__subpackages__"],
-    deps = ["//testing:friend_test"],
+    deps = [
+        "//testing:friend_test",
+        "@com_google_absl//absl/base:nullability",
+    ],
 )
 
 mozc_cc_test(

src/base/container/freelist.h

@@ -37,6 +37,7 @@
 #include <utility>
 #include <vector>
 
+#include "absl/base/nullability.h"
 #include "testing/friend_test.h"
 
 namespace mozc {
@@ -96,7 +97,7 @@ class FreeList {
     next_in_chunk_ = std::numeric_limits<size_type>::max();
   }
 
-  T* Alloc() {
+  absl::Nonnull<T*> Alloc() {
     if (next_in_chunk_ >= chunk_size_) {
       next_in_chunk_ = 0;
       // Allocate the chunk with the allocate and delay the constructions until
@@ -105,7 +106,7 @@ class FreeList {
     }
 
     // Default construct T.
-    T* ptr = pool_.back() + next_in_chunk_++;
+    absl::Nonnull<T*> ptr = pool_.back() + next_in_chunk_++;
     allocator_traits::construct(allocator_, ptr);
     return ptr;
   }

src/converter/BUILD.bazel

@@ -187,9 +187,11 @@ mozc_cc_library(
         "//dictionary:suppression_dictionary",
         "//prediction:suggestion_filter",
         "//request:conversion_request",
+        "@com_google_absl//absl/base:nullability",
         "@com_google_absl//absl/log",
         "@com_google_absl//absl/log:check",
         "@com_google_absl//absl/strings",
+        "@com_google_absl//absl/types:span",
     ],
 )