eval() may introduce a scope which needs to be represented as a context at
runtime, e.g.,

  eval('var x; let y; ()=>y')

introduces a variable y which needs to have a context allocated for it. However,
when traversing upwards to find the declaration context for a variable which leaks,
as the declaration of x does above, this context has to be understood to not be
a declaration context in sloppy mode.

This patch makes that distinction by introducing a different map for eval-introduced
contexts. A dynamic search for the appropriate context will continue past an eval
context to find the appropriate context. Marking contexts as eval contexts rather
than function contexts required updates in each compiler backend.

BUG=v8:5295, chromium:648719

Review-Url: https://codereview.chromium.org/2435023002
Cr-Commit-Position: refs/heads/master@{#41869}