Use textEdit for completions

Rather than just inserting text for completions, use textEdit to insert
text which replaces the entire identifier, including text after the
cursor. Does not remove insertText, so that LSP clients without textEdit
support can fall-back to the old behaviour -- otherwise, insertText
should be ignored.

src/messages/text_document_completion.cc

@@ -390,7 +390,7 @@ struct Handler_TextDocumentCompletion : MessageHandler {
       QueueManager::WriteStdout(kMethodType, out);
     } else {
       ClangCompleteManager::OnComplete callback =
-          [this, request, existing_completion, is_global_completion,
+          [this, request, existing_completion, end_pos, is_global_completion,
            has_open_paren](const lsRequestId& id,
                            std::vector<lsCompletionItem> results,
                            bool is_cached_result) {
@@ -402,6 +402,13 @@ struct Handler_TextDocumentCompletion : MessageHandler {
             FilterAndSortCompletionResponse(&out, existing_completion,
                                             has_open_paren,
                                             g_config->completion.filterAndSort);
+            // Add text edits with the same text, but whose ranges include the
+            // whole token from start to end.
+            for (auto& item : out.result.items) {
+              item.textEdit = lsTextEdit{
+                  lsRange(request->params.position, end_pos), item.insertText};
+            }
+
             QueueManager::WriteStdout(kMethodType, out);
 
             // Cache completion results.