[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
1 file changed