[clang-tools-extra] [clangd] Add an early initialization hook to FeatureModule (PR #225198)

[Aleksandr Platonov via cfe-commits]

ArcsinX wrote:

Friendly ping.

This PR is just an oversight related to #221054, with only +2 lines diff (+ tests). Without any changes in clangd behavior. Currently this blocks me in continuation of moving clang-tidy into feature module.

https://github.com/llvm/llvm-project/pull/225198