After looking at implementing the existing Private Fields support for Ion, I came to the conclusion that what I'd built was too complex and didn't share enough functionality with our pre-existing codebase.

Given my increasing level of comfort with the front end, I was able to find a new design that's appreciably simpler, while also easing the (upcoming) implementation of both private methods and ergnomic brand checks.

Adds CheckPrivateField, and uses it to guard private field access before
calling regular element ops, rather than having custom element ops

CheckPrivateField takes two arguments: A ThrowCondition, a message, then using
the top two elements of the stack determines if a throw is required, and if it
is not, places on top of the stack whether or not the field exists.

This design is nice because it means that we'll be able to easily use this
opcode for implementing private methods and ergonomic brand checks for private
fields.

This patch implements the new opcode, deletes the old PrivateElem ops,
and provides Baseline callVM support. It's future work to provide IC, Ion and
Warp support.

https://hg.mozilla.org/mozilla-central/rev/aa11e196e0c7

A non-unified build warns about CanAttachPrivateInitIC being unused. Also \o/