Original CL: https://crrev.com/c/1018468

During code generation, we generate self-references (i.e. references to
the Code object currently being generated) as references to a temporary
handle. When the final Code object has been allocated, the handle's
location is fixed up and RelocInfo iteration fixes up all references
embedded in the generated code.

This adds support for this mechanism to the builtins constants table
builder. CodeObject() is now a new handle pointing to a dedicated
self-reference marker in order to distinguish between self-references
and references to undefined. In Factory::NewCode, we patch up
the constants table.

TBR=yangguo@chromium.org,mlippautz@chromium.org

Bug: v8:6666
Change-Id: I3fa422c57de99c9851dc7a86394a8387c7c2b397
Reviewed-on: https://chromium-review.googlesource.com/1039366
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52916}