C++ implicit members should be implicit

[zrlk]

Right now we index (approximately; these members don't get created unless used):

//- @Foo defines/binding ClassFoo
//- @Foo defines/binding CtorFoo
class Foo {
};

CtorFoo is an otherwise regular looking function node that shares the anchor location @Foo. It should probably be marked as implicit.

[shahms]

defines/binding/implicit ?

[zrlk]

We have defines/implicit, and that's documented to work for the implicit destructor (!), but isn't being used for this case.