Helix comes with support for clangd and clang-format out of the box! However, you also need to configure the clangd server to not insert headers improperly. To do this, create a .helix/languages.toml file in the project root:
[language-server.ladybird]
command = "clangd"
args = ["--header-insertion=never"]
[[language]]
name = "cpp"
language-servers = ["ladybird"]