[clang-tools-extra] [clangd] Add languages as server capabilities (PR #75633)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

> What about a slightly different take? What if in addition to adding `clangdLanguages` (without HLSL) we also add `clangdExperimentalFeatures` and add `HLSL` there. The problem I'm trying to solve here is to avoid someone enabling HLSL on a language server that doesn't advertise that it has support.

That sounds fine to me!

https://github.com/llvm/llvm-project/pull/75633