We often only need to store information about declared (i.e.
non-imported) functions in a wasm module. Thus we remove the number of
imported function from a function index. We do this in several places,
with different amount of checking.

This CL extracts this logic to a new {declared_function_index} helper
and uses it wherever we do this translation. This more or less
establishes the concept of "declared function index" and hopefully
prevents errors in the future.

R=jkummerow@chromium.org

Bug: chromium:1045767
Change-Id: I7e957401495a2a8cb5d2c51031f9c69fe46195d8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2020763
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65994}