- Different places used is_simple to mean different things; renamed one.

- No need to do Scope::SetHasNoSimpleParameters multiple times.

- Normally we create VAR parameters with a name, or (for destructuring
  parameters), TEMPORARY parmeters with an empty name. *Except* for
  destructuring rest parameters; then we create VAR a parameter with an empty
  name. This CL makes the empty-named parameter TEMPORARY instead of VAR.

- This makes it clear that Parser::DeclareFormalParameters declares exactly
  those params which Parser::BuildParamerterInitializationBlock doesn't declare.

- This unification doesn't change any functionality, but it makes sense to do
  since I'll need to make PreParser emulate what Parser does; this way I don't
  need to emulate the weird behavior.

BUG=v8:5501

Change-Id: Ifa6c116bc5908f4e03a36e74f47558888d1582bd
Reviewed-on: https://chromium-review.googlesource.com/443106Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43220}