[clangd] Polish clangd/inlayHints and expose them by default.

This means it's a "real feature" in clangd 14, albeit one that requires special
client support.

- remove "preview" from the flag description
- expose the `clangdInlayHints` capability by default
- provide `position` as well as `range`
- support `InlayHintsParams.range` to restrict the range retrieved
- inlay hint list is in document order (sorted by position)

Still to come: control feature via config rather than flag.

Fixes https://github.com/clangd/clangd/issues/313
Protocol doc is in https://github.com/llvm/clangd-www/pull/56/files

Differential Revision: https://reviews.llvm.org/D116699