The only remaining use of this VariableMode is for the names of sloppy
named function expressions. This patch instead uses CONST for such
bindings (just as we do in strict mode) and instead marks those
Variables specially. During code generation a new helper method,
Variable::throw_on_const_assignment(), is called to decide whether
to throw or silently ignore the assignment.

Review-Url: https://codereview.chromium.org/2233673003
Cr-Commit-Position: refs/heads/master@{#39052}