Port 7516fe1e

Original commit message:

    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.

R=adamk@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com, mbrandy@us.ibm.com
BUG=

Review-Url: https://codereview.chromium.org/2299043002
Cr-Commit-Position: refs/heads/master@{#39060}