[lldb] Log the actual expression result in UserExpression::Evaluate

This used to be a LLDB_LOGF call that used the printf %s syntax.
0ab109d43d9d8389fe686ee8df4a82634f246b55 changed it to LLDB_LOG but didn't
update this format string to use formatv's syntax so this just printed '%s'.

GitOrigin-RevId: e87b8e4498df3bc1fcb751dfa24816ecf1ac02bd
diff --git a/source/Expression/UserExpression.cpp b/source/Expression/UserExpression.cpp
index 5beed46..c6a288a 100644
--- a/source/Expression/UserExpression.cpp
+++ b/source/Expression/UserExpression.cpp
@@ -362,7 +362,7 @@
 
           LLDB_LOG(log,
                    "== [UserExpression::Evaluate] Execution completed "
-                   "normally with result %s ==",
+                   "normally with result {0} ==",
                    result_valobj_sp->GetValueAsCString());
         } else {
           LLDB_LOG(log, "== [UserExpression::Evaluate] Execution completed "