[clang-tidy] google-readability-function: skip std::nullptr_t

Parameters of type std::nullptr_t can only have one value, so it doesn't make
sense to name them.


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@221340 91177308-0d34-0410-b5e6-96231b3b80d8