[clangd] Use empty string to represent None semantics in FoldingRange::kind

clang-tools-extra/clangd/Protocol.h

bool fromJSON(const llvm::json::Value &, FoldingRangeParams &,
               llvm::json::Path);
 
 /// Stores information about a region of code that can be folded.
 struct FoldingRange {
   unsigned startLine = 0;
   unsigned startCharacter;
   unsigned endLine = 0;
   unsigned endCharacter;
-  llvm::Optional<std::string> kind;
+  std::string kind;
 };
 llvm::json::Value toJSON(const FoldingRange &Range);
 
 /// Keys starting with an underscore(_) represent leaves, e.g. _total or _self
 /// for memory usage of whole subtree or only that specific node in bytes. All
 /// other keys represents children. An example:
 ///   {
 ///     "_self": 0,

clang-tools-extra/clangd/Protocol.cpp

llvm::json::Value toJSON(const FoldingRange &Range) {
   llvm::json::Object Result{
       {"startLine", Range.startLine},
       {"endLine", Range.endLine},
   };
   if (Range.startCharacter)
     Result["startCharacter"] = Range.startCharacter;
   if (Range.endCharacter)
     Result["endCharacter"] = Range.endCharacter;
-  if (Range.kind)
-    Result["kind"] = *Range.kind;
+  if (!Range.kind.empty())
+    Result["kind"] = Range.kind;
   return Result;
 }
 
 llvm::json::Value toJSON(const MemoryTree &MT) {
   llvm::json::Object Out;
   int64_t Total = MT.self();
   Out["_self"] = Total;
   for (const auto &Entry : MT.children()) {