The function InstanceBuilder::ProcessImports got long and hard to
understand. This CL is the third change of a series which aims at
extracting parts of ProcessImports into specialized functions.

This CL extracts the processing of imported memory into a separate
function. Aside from copying the code, I made the following changes:
* Rename {index} to {import_index}.
* Add one `return false;` after reporting a LinkError.
  - I think it should have been this way all along.

R=clemensh@chromium.org

Bug: v8:8562
Change-Id: I34d68d7bc1013c2743a68778dfc22a2d8e1d46af
Reviewed-on: https://chromium-review.googlesource.com/c/1382731
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58346}