[clang-tools-extra] [clangd] Add `HeaderInsertion` yaml config option (PR #128503)

[via cfe-commits]

MythreyaK wrote:

> One issue that I see with the current implementation is that it makes the command-line flag `--header-insertion=never` have no effect.

Oh right! I completely forgot about that. 

> To get the command line flag to interact with the config option better, we need to handle it in [`FlagsConfigProvider`](https://searchfox.org/llvm/rev/107aa6a3d3ab96b7eec55e1ec5c3eabfa6ab2f9f/clang-tools-extra/clangd/tool/ClangdMain.cpp#655), which gives the command-line flag the ability to influence the value of `Config::current().HeaderInsertion.Policy`.
> 
> We have a few options regarding the precedence, but I think the simplest to implement is to handle it similar to --background-index [here](https://searchfox.org/llvm/rev/107aa6a3d3ab96b7eec55e1ec5c3eabfa6ab2f9f/clang-tools-extra/clangd/tool/ClangdMain.cpp#711-713): if the flag's value is NeverInsert, then write that into the config object, otherwise leave it alone.

Clarification, so if the user has `IWYU` in CLI, but `never` in the config, `never` is what will be used? 

> Since header insertion is a side effect of accepting of code completion proposal, it would make sense to have this option under `Completion` rather than at the top level.

Sounds good, I'll update my patch shortly. 

https://github.com/llvm/llvm-project/pull/128503