don't automatically initialise all internal slots to undefined

[michaelficarra]

We need to clean up how we create objects and set initial values of their internal slots. It is a common pattern today to call OrdinaryCreateFromConstructor with a list of additional slot names and then in the immediately following steps, set the initial values of each of these additional fields. But technically according to 6.1.7.2 Object Internal Methods and Internal Slots, these slots are given the value undefined in the interim.

Unless specified otherwise, the initial value of an internal slot is the value undefined.

This means that the declared type of each of these internal slots is implicitly unioned with undefined. We should instead create these slots and give them their initial values atomically, then remove this provision.

See related #3383.

[syg]

Recap of editor call:

• Let's change MakeBasicObject to take anonymous records of { [[SlotName]]: initialValue } "templates"
• Do the exercise of changing the callers to use the new form, which hopefully naturally points out where currently the initialization is already too far away from the allocation

[michaelficarra]

And of course remove the implicit initialisation affordance.

[bakkot]

MakeBasicObject is used upstream, at least in WebIDL but plausibly also other specs, so this will take at least a little integration work.