[lldb-dap] Show hidden frames as "subtle"

This commit takes advantage of the recently introduced
`SBFrame::IsHidden` to show those hidden frames as "subtle" frames in
the UI. E.g., VS Code renders such frames grayed out in the stack trace

Author: vogelsgesang

lldb/tools/lldb-dap/JSONUtils.cpp

@@ -763,6 +763,9 @@ llvm::json::Value CreateStackFrame(lldb::SBFrame &frame) {
     object.try_emplace("instructionPointerReference", formatted_addr);
   }
 
+  if (frame.IsArtificial() || frame.IsHidden())
+    object.try_emplace("presentationHint", "subtle");
+
   return llvm::json::Value(std::move(object));
 }