Refactoring only. Cleanup some loose ends from #10196.

Initially the idea was to only allow modifier letters at the end of
identifiers. Since we later decided to allow modifier letters also in
the middle of identifiers (because not doing so would not fix the
regression completely), the names `suffix` and `okIdSuffixChar` don't
seem appropriate anymore.

Remove TODO. Move test from should_fail to should_compile.