[clangd] Dont display `<unknown>` kinds in hover board

[kadircet]

Summary

Currently when hovering over an auto or decltype that resolve to a
builtin-type, clangd would display <unknown> as the kind of the symbol.

Drop that to make rendering nicer.

[merge_guards_bot]

Unit tests: pass. 61895 tests passed, 0 failed and 782 were skipped.

clang-tidy: unknown.

clang-format: pass.

Build artifacts: diff.json, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

clang-tools-extra/clangd/Hover.cpp

markup::Document HoverInfo::present() const {
   // function `foo`
   //
   // expression
   //
   // Note that we are making use of a level-3 heading because VSCode renders
   // level 1 and 2 headers in a huge font, see
   // https://github.com/microsoft/vscode/issues/88417 for details.
   markup::Paragraph &Header = Output.addHeading(3);
+  if (Kind != index::SymbolKind::Unknown)
     Header.appendText(index::getSymbolKindString(Kind));
   assert(!Name.empty() && "hover triggered on a nameless symbol");
   Header.appendCode(Name);
 
   // Put a linebreak after header to increase readability.
   Output.addRuler();
   // Print Types on their own lines to reduce chances of getting line-wrapped by
   // editor, as they might be long.
   if (ReturnType) {

clang-tools-extra/clangd/unittests/HoverTests.cpp

struct {
     const std::function<void(HoverInfo &)> Builder;
     llvm::StringRef ExpectedRender;
   } Cases[] = {
       {
           [](HoverInfo &HI) {
             HI.Kind = index::SymbolKind::Unknown;
             HI.Name = "X";
           },
-          R"(<unknown> X)",
+          R"(X)",
       },
       {
           [](HoverInfo &HI) {
             HI.Kind = index::SymbolKind::NamespaceAlias;
             HI.Name = "foo";
           },
           R"(namespace-alias foo)",
       },