Declare Variables with a name and position, rather than by passing
through a VariableProxy. This allows us to not create dummy proxies
for things like function declarations, and allows us to consider those
declarations unused.

As a side-effect, we also have to check if a variable is unused in the
bytecode generator (as it will no longer be allocated), and we end up
skip generating code/SFIs for dead variables/functions.

Change-Id: I4c2c872473f23e124f9456b4b92f87159658f8e0
Reviewed-on: https://chromium-review.googlesource.com/c/1414916
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59088}