Storing a VariableProxy in declarations means that a declaration and
initialisation assignment are tightly coupled to use the same var. In
particular, this means that Var declarations in with scopes have to
clone the VariableProxy to split the declaration and initializer LHS
lookup.

This patch changes declarations to point directly to the Variable, not
the VariableProxy. This will allow future refactoring to decouple
declarations and initialisations.

Change-Id: I0baa77bfd12fe175f9521d292740d7d712cffd37
Reviewed-on: https://chromium-review.googlesource.com/c/1406683Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58843}