Construction of JSFunction objects is complex, mostly due to the
existence of multiple functions kinds (JS, wasm, builtin, test, ...)
that are all created slightly differently. For example, JS functions
may come with an existing FeedbackCell (and FeedbackVector), while
builtins and wasm functions always use the many_closures_cell (without
a vector).

Prior to this CL, construction logic was scattered over a family of
7 functions, without a clearly defined chokepoint for header
initialization. This was hard to understand, hard to modify, and
needlessly inefficient (by setting some fields twice).

This CL fixes all that by introducing JSFunctionBuilder. The BuildRaw
method is the chokepoint for allocation and initialization, and Build
performs common pre- and post-work.

Future work:
- Remove now-deprecated functions.
- Untangle SFI/Map/JSFunction construction and remove
  Factory::NewFunction and NewFunctionArgs.

Bug: v8:8888
Change-Id: I709a2a44ee02e10593a4c9afe43d4d2c6d6351c4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2527098Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71084}