This fixes a corner-case in redeclaration handling, where the ES2015
early error case got mixed up with legacy const handling in the parser.
Redeclaration using ES2015 'let' and 'const' should be early errors,
but legacy 'const' redeclaration has historically been a runtime error,
and should stay that way until legacy 'const' is gone.
The fix here is uglier than it might be due to
https://code.google.com/p/v8/issues/detail?id=4577, which keeps us
from simplifying the mess of if/else-if in the current code.
BUG=v8:4576
LOG=n
Review URL: https://codereview.chromium.org/1485943002
Cr-Commit-Position: refs/heads/master@{#32429}
