The spec calls out to Promise.prototype.then and also passes around
the constructor of the receiver to Promise.prototype.finally.

Adds a new constructor slot to PromiseFinallyContext enum and this is
used to create a new promise in the thenFinally/catchFinally callbacks.

Created a new PromiseResolve TFS builtin refactored from
the existing PromiseResolve builtin. PromiseResolveWrapper
calls out to this TFS Builtin and is now exposed as Promise.resolve.
The thenFinally and catchFinally callbacks also call out to the
PromiseResolve TFS builtin.

Spec -- https://tc39.github.io/proposal-promise-finally/

Bug: v8:5967
Change-Id: I2ce89f14d3b149619d11e424b6e37062e466c4d5
Reviewed-on: https://chromium-review.googlesource.com/652026Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47898}