Make llvm::StringRef to std::string conversions explicit.

This is how it should've been and brings it more in line with
std::string_view. There should be no functional change here.

This is mostly mechanical from a custom clang-tidy check, with a lot of
manual fixups. It uncovers a lot of minor inefficiencies.

This doesn't actually modify StringRef yet, I'll do that in a follow-up.

lldb/source/Interpreter/OptionGroupPythonClassWithDict.cpp

@@ -91,13 +91,13 @@ Status OptionGroupPythonClassWithDict::SetOptionValue(
   switch (option_idx) {
   case 0:
   case 3: {
-    m_name.assign(option_arg);
+    m_name.assign(std::string(option_arg));
   } break;
   case 1: {
       if (!m_dict_sp)
         m_dict_sp = std::make_shared<StructuredData::Dictionary>();
       if (m_current_key.empty())
-        m_current_key.assign(option_arg);
+        m_current_key.assign(std::string(option_arg));
       else
         error.SetErrorStringWithFormat("Key: \"%s\" missing value.",
                                         m_current_key.c_str());