ConvertToInt should take an explicit argument for its conversion approach

[Ms2ger]

What is the issue with the Web IDL Standard?

Currently, there's prose like

If the conversion is to an IDL type associated with the [EnforceRange] extended attribute, then:

which couples it too tightly to its callers. It would be better if its callers, which actually have an IDL type on hand, pass an argument to choose how to handle out-of-range values.

Noticed in review of WebAssembly/memory64#86