Verify that IR keywords never interfere with identifiers

There are a few keywords that the ir-scanner defines

https://github.com/google/xls/blob/e09727b380251232906c182ec66d8be85f590ff8/xls/ir/ir_scanner.h#L65-L71

Some of them such as `top` or `file_number` look like they could also be used commonly as identifiers. And in fact, IR functions with such names will fail to parse; currently this is (accidentally?) working because identifiers typically have some prefix attached (such as the package name).

This issue is to verify that we never actually get into a situation to have a clash between keywords and identifier (and potentially fix it if so).