Multiple dfns with same text but different data-dfn-fors number their ids

[jyasskin]

Description of problem

URL to affected spec or repo: w3c/hr-time#156

<dfn data-export="" data-dfn-for="wall clock">unsafe current time</dfn>

produces https://w3c.github.io/hr-time/#dfn-unsafe-current-time-0, and

<dfn data-export="" data-dfn-for="monotonic clock">unsafe current time</dfn>

produces https://w3c.github.io/hr-time/#dfn-unsafe-current-time-1. The ids should instead include the data-dfn-for value since that's how specifications disambiguate. For backward-compatibility, this should probably only happen if a spec includes two definitions with the same text and different for values.