clangd can take arguments also via environment variable CLANGD_FLAGS.
This has to be respected in the wrapper, which checks whether a
--query-driver was provided.
$CPATH is taken to mean the same thing as -I by clang, whereas
$C_INCLUDE_PATH is -isystem. Using $CPATH means tools like clangd will
generate warnings for problems in system headers, which is undesirable.
See llvm/llvm-project#154015
This is necessary after https://github.com/NixOS/nixpkgs/pull/445095,
which moved libc++ to -cxx-isystem to improve compatibility with build
systems that invoke `clang` to compile C++ code.
