Function declarations reference a previously defined or imported
signature. Make this visible when declaring empty functions.
Also rename IMPORT_SIG_INDEX to SIG_INDEX since it can also reference a
locally defined signature.

This is the third CL in a series to improve our module decoder tests and
make them more readable.

R=titzer@chromium.org

Bug: v8:8238
Change-Id: Ibfd9ea39ea35bacdb453602f8985fb3306455de4
Reviewed-on: https://chromium-review.googlesource.com/c/1282958Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56714}