[clangd] Support textDocument/semanticTokens/edits

Summary:
This returns incremental highlights as a set of edits against the
previous highlights.

Server-side, we compute the full set of highlights, this just saves
wire-format size.

For now, the diff used is trivial: everything from the first change to
the last change is sent as a single edit.

The wire format is grungy - the replacement offset/length refer to
positions in the encoded array instead of the logical list of tokens.
We use token-oriented structs and translating to LSP forms when serializing.
This departs from LSP (but is consistent with semanticTokens today).

Tested in VSCode insiders (with a patched client to enable experimental
features).

Reviewers: hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, mgrang, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77225