[clang-tools-extra] [llvm] Add bazel support for clangd as a library. (PR #81556)

[Fangrui Song via cfe-commits]

https://github.com/MaskRay approved this pull request.

bazel-specific changes often have a title `[bazel] Add clangd support`. Please consider following suit.

It would be nice to have a fuller support, but I understand having this cc_library first can make your downstream changes easier, so LGTM.

https://github.com/llvm/llvm-project/pull/81556