Move the logic for allocating the global declaration pair array
from VisitDeclarations to a later step. This is required for
concurrent bytecode generation.

This change requires adding support for reserving fixed
constant pool array entries, which can be later updated
with the value of the literal.

BUG=v8:5203

Review-Url: https://codereview.chromium.org/2167763003
Cr-Commit-Position: refs/heads/master@{#38010}