Use a new lexical context for sloppy-mode eval

Use a new lexical context for sloppy-mode eval

In ES6, direct eval() in sloppy mode uses the enclosing function-level
("var") scope for var-style bindings and a new lexical scope for lexical
bindings like let and class. This patch implements that feature by making
lexical bindings that are directly within an EVAL_SCOPE be on the local
scope rather than the enclosing one.

BUG=v8:4288
LOG=Y
R=adamk

Review URL: https://codereview.chromium.org/1274193004

Cr-Commit-Position: refs/heads/master@{#30120}