[lldb][NFCI] Avoid construction of temporary std::strings in Variable

[bulbazord]

Summary

A common thing to do is to call str().c_str() to get a null-terminated
string out of an existing StringRef. Most of the time this is to be able
to use a printf-style format string. However, llvm::formatv can handle
StringRefs without the need for the additional allocation. Using that
makes more sense.

[JDevlieghere]

LGTM

lldb/source/Symbol/Variable.cpp

Status Variable::GetValuesForVariableExpressionPath(
   } break;
 
   default: {
     static RegularExpression g_regex(
         llvm::StringRef("^([A-Za-z_:][A-Za-z_0-9:]*)(.*)"));
     llvm::SmallVector<llvm::StringRef, 2> matches;
     variable_list.Clear();
     if (!g_regex.Execute(variable_expr_path, &matches)) {
-      error.SetErrorStringWithFormat(
-          "unable to extract a variable name from '%s'",
-          variable_expr_path.str().c_str());
+      error.SetErrorStringWithFormatv(
+          "unable to extract a variable name from '{0}'", variable_expr_path);
       return error;
     }
     std::string variable_name = matches[1].str();
     if (!callback(baton, variable_name.c_str(), variable_list)) {
       error.SetErrorString("unknown error");
       return error;
     }
     uint32_t i = 0;
while (i < variable_list.GetSize()) {
       }
 
       llvm::StringRef variable_sub_expr_path =
           variable_expr_path.drop_front(variable_name.size());
       if (!variable_sub_expr_path.empty()) {
         valobj_sp = variable_valobj_sp->GetValueForExpressionPath(
             variable_sub_expr_path);
         if (!valobj_sp) {
-          error.SetErrorStringWithFormat(
-              "invalid expression path '%s' for variable '%s'",
-              variable_sub_expr_path.str().c_str(),
-              var_sp->GetName().GetCString());
+          error.SetErrorStringWithFormatv(
+              "invalid expression path '{0}' for variable '{1}'",
+              variable_sub_expr_path, var_sp->GetName().GetCString());
           variable_list.RemoveVariableAtIndex(i);
           continue;
         }
       } else {
         // Just the name of a variable with no extras
         valobj_sp = variable_valobj_sp;
       }