Index contains no normative definition of "user-declared" CWG3148

[Quuxplusone]

https://eel.is/c++draft/dcl.fct.def.default#def:user-provided says:

A function is user-provided if it is user-declared and not explicitly defaulted or deleted on its first declaration.

However, there is no normative definition of user-declared, at least not according to the Index. The Index currently jumps straight from "usable in constant expressions" to "user-defined conversion sequence."

This is probably an easy fix, just adding an index entry somewhere.

[t3nsor]

[CWG2799] contains the intended definition, but no one knows how to actually resolve that issue. It seems that the definition should be split out into a separate issue.

[jensmaurer]

Created CWG3148 for the definition of "user-declared".