[clang-tools-extra] [clang-tidy][NFC] Fix llvm-prefer-static-over-anonymous-namespace warnings 3/N (PR #164085)

[Victor Chernyakin via cfe-commits]

https://github.com/localspook approved this pull request.

Couple of suggested changes:
- `isTokAtEndOfExpr` (`RedundantExpressionCheck.cpp`, line 811) should be made `static`
- `CastSequence` (`UseNullptrCheck.cpp`, line 24) could be made static and moved out of the anonymous namespace

(Sorry that I'm not doing easy-to-apply GitHub suggestions; seems you can only add those around changed lines)

https://github.com/llvm/llvm-project/pull/164085