[clangd] Log warning when using legacy (theia) semantic highlighting.

[sammccall]

Summary

The legacy protocol will be removed on trunk after the 12 branch cut,
and gone in clangd 13.

[kadircet]

LGTM, a friendly ping to @nridge though, as he was working on theia related stuff (at least in the past, not sure if you still do :D), to give him a heads-up if this is going to require some changes on their end.

[nridge]

Thanks for the heads up! It would be nice to get D77702 landed before the removal of TheiaSemanticHighlighting. I'll try to find some time to rebase it.

[sammccall]

In D95031#2509896, @nridge wrote:

Thanks for the heads up! It would be nice to get D77702 landed before the removal of TheiaSemanticHighlighting. I'll try to find some time to rebase it.

Good point!
I've revived D77811, as I'd prefer not to start adding modifiers before can make them independent of kinds. (Moving features to different infrastructure tends to involve some backward-compatibility/testing toil...)

From there I think it should be easy to get the rest of this merged:

• scope modifiers make a lot of sense and should be straightforward to add
• dependent type vs name: maybe we should consider making dependent a modifier instead? but otherwise LG
• the "member" issue I think has been resolved for the most part, by clarifying the spec

clang-tools-extra/clangd/ClangdLSPServer.cpp

void ClangdLSPServer::onInitialize(const InitializeParams &Params,
   Opts.TheiaSemanticHighlighting =
       Params.capabilities.TheiaSemanticHighlighting;
   if (Params.capabilities.TheiaSemanticHighlighting &&
       Params.capabilities.SemanticTokens) {
     log("Client supports legacy semanticHighlights notification and standard "
         "semanticTokens request, choosing the latter (no notifications).");
     Opts.TheiaSemanticHighlighting = false;
   }
+  if (Opts.TheiaSemanticHighlighting) {
+    log("Using legacy semanticHighlights notification, which will be removed "
+        "in clangd 13. Clients should use the standard semanticTokens "
+        "request instead.");
+  }
 
   if (Params.rootUri && *Params.rootUri)
     Opts.WorkspaceRoot = std::string(Params.rootUri->file());
   else if (Params.rootPath && !Params.rootPath->empty())
     Opts.WorkspaceRoot = *Params.rootPath;
   if (Server)
     return Reply(llvm::make_error<LSPError>("server already initialized",
                                             ErrorCode::InvalidRequest));