Like so:
# C++
# Clangd from clang-tools must come first.
(hiPrio clang-tools.override {
llvmPackages = llvmPackages_16;
enableLibcxx = false;
})
# Do not use the clangd from this package as it does not work correctly with
# stdlib headers.
llvmPackages_16.libstdcxxClang