[clang-tidy] Reenable google-readability-casting.

The "no-op suggested fixes" issue has been fixed in https://github.com/llvm/llvm-project/pull/71650 and no longer raises false positives on the code referenced in https://issues.chromium.org/issues/40209512 , so it should be sound to re-enable this clang-tidy warning.

Bug: 40209512
Change-Id: I8b9464f4c5b8b48705d5dd46ba6ad762b847c442
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6341202
Commit-Queue: Julia Hansbrough <flowerhack@google.com>
Reviewed-by: Rick Byers <rbyers@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1432258}

.clang-tidy

@@ -1,6 +1,4 @@
 ---
-  # TODO(crbug.com/1282228): reenable google-readability-casting once it no
-  # longer has as many false-positives.
   Checks:          '-*,
                     bugprone-argument-comment,
                     bugprone-assert-side-effect,
@@ -19,6 +17,7 @@
                     google-default-arguments,
                     google-explicit-constructor,
                     google-objc-avoid-nsobject-new,
+                    google-readability-casting,
                     google-upgrade-googletest-case,
                     misc-misleading-identifier,
                     misc-homoglyph,