The CSA implementation is a separate handler so that TF has the
opportunity to reduce to a direct call, skipping some of the dispatching
in the CloneObjectIC stub.

This patch moves the looping over a source object's keys and values into the
base CodeStubAssembler, so that it can be shared between ObjectAssignFast
and CloneObjectIC_Slow.

During each step of the loop, storing is delegated to a new SetPropertyInLiteral
helper in KeyedStoreGenericGenerator, which performs a store without consulting
the prototype chain, and automatically reconfigures accessors into data
properties regardless of their attributes.

BUG=v8:8067, v8:7611
R=ishell@chromium.org, jkummerow@chromium.org

Change-Id: I06ae89f37e9b4265aab67389cf68a96529f90578
Reviewed-on: https://chromium-review.googlesource.com/1182122
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55806}