[clang-tools-extra] [clangd] Allow specifying what headers are always included via "" or <> (PR #67749)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

> I don't have any energy and the massive reachitecting requested would take me tons of time.

I certainly didn't intend to request a massive rearchitecting. Sam's comment gave me the impression that hooking up the preferences to include-cleaner would be fairly straightforward.

Anyways, Sam also said:

> Landing this without include-cleaner support is OK with me

so I'm fine with leaving that to a future enhancement.

@kleinesfilmroellchen could you rebase the patch to a recent `main` please (or merge `main` into it)? As far as I can tell, all other comments have been addressed, so if the patch has a green buildkite run on top of recent `main`, I'm happy to merge it without further changes.

https://github.com/llvm/llvm-project/pull/67749