[clangd] Add add commit characters to the server capabilities

Summary:
Make it more convinient for the clients to select completion items by
providing a set of default characters (punctuation).

Related issue: https://github.com/clangd/clangd/issues/284

Reviewers: sammccall

Reviewed By: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74606

clang-tools-extra/clangd/ClangdLSPServer.cpp

@@ -559,6 +559,7 @@ void ClangdLSPServer::onInitialize(const InitializeParams &Params,
             {"codeActionProvider", std::move(CodeActionProvider)},
             {"completionProvider",
              llvm::json::Object{
+                 {"allCommitCharacters", " \t()[]{}<>:;,+-/*%^&#?.=\"'|"},
                  {"resolveProvider", false},
                  // We do extra checks for '>' and ':' in completion to only
                  // trigger on '->' and '::'.

clang-tools-extra/clangd/Protocol.h

@@ -1101,6 +1101,8 @@ struct CompletionItem {
   /// the client side.
   float score = 0.f;
 
+  // TODO: Add custom commitCharacters for some of the completion items. For
+  // example, it makes sense to use () only for the functions.
   // TODO(krasimir): The following optional fields defined by the language
   // server protocol are unsupported:
   //

clang-tools-extra/clangd/test/initialize-params.test

@@ -7,6 +7,7 @@
 # CHECK-NEXT:    "capabilities": {
 # CHECK-NEXT:      "codeActionProvider": true,
 # CHECK-NEXT:      "completionProvider": {
+# CHECK-NEXT:        "allCommitCharacters": " \t()[]{}<>:;,+-/*%^&#?.=\"",
 # CHECK-NEXT:        "resolveProvider": false,
 # CHECK-NEXT:        "triggerCharacters": [
 # CHECK-NEXT:          ".",