Currently, Declaration stores a Scope pointer to whichever Scope the
declaration appeared in. This is used to disallow var declarations
being hoisted over lexical declarations. For example:

  {
    let x;
    { var x; }
  }

But in fact this is the only sort of case where storing the scope
is required: for lexical declarations (including function declarations
appearing in blocks), Declaration::scope() was always identical to
Declaration::proxy()->var()->scope(). That is, only var declarations
end up "nested" in this way.

This patch adds a subclass of VariableDeclaration to store the Scope.
Since the only thing that cares about that data is Scope analysis,
this isn't treated as a distinct AstNode::NodeType from VariableDeclaration,
leaving all AstVisitors untouched in the process.

Also reworked the logic in Scope::CheckConflictingVarDeclarations() for
clarity after making changes to accomodate the new code.

Change-Id: I6ee4298700508ab9e28a76ddb8504bae68bc473f
Reviewed-on: https://chromium-review.googlesource.com/619595
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47441}