Add the 'start' and 'length' to the completions response.

[ivanhernandez13]

Summary

Add the 'start' and 'length' to the completions response. This will fix an issue where applying a completion 'foo.bar' to 'po foo.b' inserts the entire completion resulting in 'po foo.foo.bar'.

[labath]

This needs a test case, but I am also not sure that the implementation is correct even.
What will this do with multiword commands "frame variable foo.b<TAB>" ? Or even single-word commands that have multiple arguments ("vo foo bar foo.b<TAB>")?

lldb/tools/lldb-vscode/lldb-vscode.cpp

void request_completions(const llvm::json::Object &request) {
   lldb::SBStringList matches;
   lldb::SBStringList descriptions;
   g_vsc.debugger.GetCommandInterpreter().HandleCompletionWithDescriptions(
     text.c_str(),
     actual_column,
     0, -1, matches, descriptions);
   size_t count = std::min((uint32_t)50, matches.GetSize());
   targets.reserve(count);
+  auto command_prefix = llvm::StringRef(text).rsplit(" ").second;
   for (size_t i = 0; i < count; i++) {
     std::string match = matches.GetStringAtIndex(i);
     std::string description = descriptions.GetStringAtIndex(i);
 
     llvm::json::Object item;
     EmplaceSafeString(item, "text", match);
     if (description.empty())
       EmplaceSafeString(item, "label", match);
     else
       EmplaceSafeString(item, "label", match + " -- " + description);
 
+    auto command_prefix_size = command_prefix.size();
+    if (command_prefix_size && match.find(command_prefix, 0) == 0) {
+      EmplaceSafeString(item, "start",
+                        llvm::StringRef(std::to_string(actual_column -
+                                                       command_prefix_size)));
+      EmplaceSafeString(item, "length",
+                        llvm::StringRef(std::to_string(command_prefix_size)));
+    }
+
     targets.emplace_back(std::move(item));
   }
 
   body.try_emplace("targets", std::move(targets));
   response.try_emplace("body", std::move(body));
   g_vsc.SendJSON(llvm::json::Value(std::move(response)));
 }