The body of a generator function can now refer to the generator's input value via a new
"function.sent" expression.  We extend the proposal at
https://github.com/allenwb/ESideas/blob/master/Generator%20metaproperty.md
in the obvious way to also apply to GeneratorResumeAbrupt.
This will enable us to desugar yield*.

The new syntax is behind a new --harmony-function-sent flag.

BUG=v8:4700
LOG=n

Review URL: https://codereview.chromium.org/1620253003

Cr-Commit-Position: refs/heads/master@{#33574}