Installing a feedback vector in a closure is a multi-step process.
The closure actually points to a Cell that points to a feedback
vector or undefined if we haven't created one yet.
This happens because we often create closures before we've compiled
the code.

JSFunction::EnsureLiterals is the bottleneck in our system that
creates a feedback vector if necessary. The predicates it used to
determine what to do were arcane. This CL allows it to think it
terms of state, and clarifies the reading of that useful
bottleneck.

I also did a few renamings in parts of the code that referred
to a "literals array," which we don't have any more.

BUG=

Review-Url: https://codereview.chromium.org/2681773004
Cr-Commit-Position: refs/heads/master@{#43035}