[clangd] Add a capability to enable completions with fixes.

[ilya-biryukov]

NIT:
I've been pushing towards giving clangd-specific extensions names that would absolutely never clash with anything that LSP proposes. In particular, the file status notification is textDocument/clangd.fileStatus.
With that in line, I propose to use clangd.editsNearCursor for the flag name. Other than that, LGTM.

clangd/ClangdLSPServer.cpp

BaseCDB = llvm::make_unique<DirectoryBasedGlobalCompilationDatabase>(
         CompileCommandsDir);
   CDB.emplace(BaseCDB.get(), Params.initializationOptions.fallbackFlags,
               ClangdServerOpts.ResourceDir);
   Server.emplace(*CDB, FSProvider, static_cast<DiagnosticsConsumer &>(*this),
                  ClangdServerOpts);
   applyConfiguration(Params.initializationOptions.ConfigSettings);
 
   CCOpts.EnableSnippets = Params.capabilities.CompletionSnippets;
+  CCOpts.IncludeFixIts = Params.capabilities.CompletionFixes;
   DiagOpts.EmbedFixesInDiagnostics = Params.capabilities.DiagnosticFixes;
   DiagOpts.SendDiagnosticCategory = Params.capabilities.DiagnosticCategory;
   DiagOpts.EmitRelatedLocations =
       Params.capabilities.DiagnosticRelatedInformation;
   if (Params.capabilities.WorkspaceSymbolKinds)
     SupportedSymbolKinds |= *Params.capabilities.WorkspaceSymbolKinds;
   if (Params.capabilities.CompletionItemKinds)
     SupportedCompletionItemKinds |= *Params.capabilities.CompletionItemKinds;

clangd/Protocol.h

struct ClientCapabilities {
   /// using the "category" extension.
   /// textDocument.publishDiagnostics.categorySupport
   bool DiagnosticCategory = false;
 
   /// Client supports snippets as insert text.
   /// textDocument.completion.completionItem.snippetSupport
   bool CompletionSnippets = false;
 
+  /// Client supports completions with additionalTextEdit near the cursor.
+  /// This is a clangd extension. (LSP says this is for unrelated text only).
+  /// textDocument.completion.editsNearCursor
+  bool CompletionFixes = false;
+
   /// Client supports hierarchical document symbols.
   bool HierarchicalDocumentSymbol = false;
 
   /// Client supports processing label offsets instead of a simple label string.
   bool OffsetsInSignatureHelp = false;
 
   /// The supported set of CompletionItemKinds for textDocument/completion.
   /// textDocument.completion.completionItemKind.valueSet

clangd/Protocol.cpp

if (auto *Completion = TextDocument->getObject("completion")) {
       }
       if (auto *ItemKind = Completion->getObject("completionItemKind")) {
         if (auto *ValueSet = ItemKind->get("valueSet")) {
           R.CompletionItemKinds.emplace();
           if (!fromJSON(*ValueSet, *R.CompletionItemKinds))
             return false;
         }
       }
+      if (auto EditsNearCursor = Completion->getBoolean("editsNearCursor"))
+        R.CompletionFixes = *EditsNearCursor;
     }
     if (auto *CodeAction = TextDocument->getObject("codeAction")) {
       if (CodeAction->getObject("codeActionLiteralSupport"))
         R.CodeActionStructure = true;
     }
     if (auto *DocumentSymbol = TextDocument->getObject("documentSymbol")) {
       if (auto HierarchicalSupport =
               DocumentSymbol->getBoolean("hierarchicalDocumentSymbolSupport"))