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

[Baranov Victor via cfe-commits]

vbvictor wrote:

> CastSequence (UseNullptrCheck.cpp, line 24) could be made static and moved out of the anonymous namespace

Yeah we can, but technically it isn't flagged rn because [option](https://clang.llvm.org/extra/clang-tidy/checks/llvm/prefer-static-over-anonymous-namespace.html#cmdoption-arg-AllowVariableDeclarations) is set to `true` by default

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