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 globals into a separate
function. Aside from copying the code, I made the following changes:
* Rename {index} to {import_index}.

This CL does minimal changes to the processing of globals itself.
That code looks quite complex at the moment. Therefore I want to do
the refactoring of that code in separate CLs, so that each CL is
smaller and more understandable.

R=clemensh@chromium.org

Bug: v8:8562
Change-Id: Ie553a2c9031cef07f0b9925144c79248084e428d
Reviewed-on: https://chromium-review.googlesource.com/c/1382732
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58347}