Whatever change has necessitated
https://github.com/NixOS/nixpkgs/pull/122044, it also broke clangd --
<clang-wrapper>/resource-root/include is no longer automagically
searched for includes, which kills pretty much any indexing since that
directory contains vital stuff like stddef.h etc.
Fix by appending the directory to CPATH & CPLUS_INCLUDE_PATH in the
clangd wrapper.
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.
Patryk Wychowaniec