Bug 1615719. Refactor how we get IDL method names for error reporting. r=peterv

The only behavior change here is that now we consistently use "Foo constructor"
to describe constructors. Before this we used "Foo" for the argument count and
arguments up to the overload resolution distinguishing argument and "Foo
constructor" for later arguments.

Differential Revision: https://phabricator.services.mozilla.com/D62346