Now that ES2015 const has shipped, in Chrome 49, legacy const declarations
are no more. This lets us remove a bunch of code from many parts of the
codebase.

In this patch, I remove parser support for generating legacy const variables
from const declarations. This also removes the special "illegal declaration"
bit from Scope, which has ripples into all compiler backends.

Also gone are any tests which relied on legacy const declarations.

Note that we do still generate a Variable in mode CONST_LEGACY in one case:
function name bindings in sloppy mode. The likely fix there is to add a new
Variable::Kind for this case and handle it appropriately for stores in each
backend, but I leave that for a later patch to make this one completely
subtractive.

Review URL: https://codereview.chromium.org/1819123002

Cr-Commit-Position: refs/heads/master@{#35002}