[std] Improve index by adding cross-references to defined terms

Consider "odr-use": We're pretty good at sticking a cross-reference to [basic.def.odr] after each mention of the term, but the index still only lists page 27, where the term is defined.

Suggestion:

• Introduce new macros \usedefn{term}{xref} and \usedefnx{term}{index-term}{xref}. They would render as term\iref{xref} and also put add an index entry for term or index-term, respectively.
• We could make the index entry bold for the location where we define a term.
• Replace all appearances of odr-use\iref{basic.def.odr} with \usedefn{odr-use}{basic.def.odr} etc.

Opinions?

Start by locating the existing \iref references for defined terms, especially odr-use and [basic.def.odr]. Review how index entries and cross-references are currently generated, then assess the proposed \usedefn and \usedefnx macros and replace the identified appearances if the approach is accepted; done means the references render correctly and index entries are added as specified.