This changes Scope::function_ (for holding the name binding
for named function expression) from a VariableDeclaration
to a Variable. No work is done when visiting this declaration,
since it's kCreatedInitialized, so we can treat it like
other function-specific variables.

This simplifies a wide variety of code, and centralizes
the logic for constructing the variable inside scopes.cc.
This may one day make it easier to eliminate the CONST_LEGACY
VariableMode.

R=neis@chromium.org, verwaest@chromium.org
BUG=v8:5209

Review-Url: https://codereview.chromium.org/2232633002
Cr-Commit-Position: refs/heads/master@{#38558}