Clang code model provides user with services: code completion,
syntactic and semantic highlighting, diagnostics, outline of symbols,
tooltips, renaming of local symbols. It's naturally depends on llvm and clang.
To make it work with nixos:
1. Add qt version of clang as build dependency.
2. Added patch for clang libc++ regexp trunk regexp not including path
like "libc++-version".
3. Fixed paths to llvm/clang libraries and includes.
4. Fixed name of clazy clang plugin.
