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.

Bug: v8:6666
Change-Id: If74ed91bb1c3b8abb20ff2f0a87d1bcd9a1b0511
Reviewed-on: https://chromium-review.googlesource.com/1018468
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52854}