[clang-tidy] Fix nested namespaces in `readability-static-definition-in-anonymous-namespace` check

The check previously inspected only the immediate parent namespace.
`static` in a named namespace within an unnamed namespace is still
redundant.
We will use `Decl::isInAnonymousNamespace()` method that traverses the
namespaces hierarchy recursively.

Differential Revision: https://reviews.llvm.org/D118010