clang-tools: New package

Creates and installs wrapper scripts around the clang-tools from
`llvmPackages.clang-unwrapped`.

Motivation
----------

Fixes #9214

Allows to install clang-tools for C++ development without also installing the
clang compiler and its tool-chain. This way it is possible to use e.g.
`clang-format` without conflicting with e.g. GCC's tool-chain, or the global
system tool-chain.