ES6 section 12.2.8.1 states that flags for regular expression literals
must be checked during parsing and invalid flags are early errors. This
change adapts the Scanner and (Pre)Parser to act according to the spec.

This is also a prerequisite to unify the handling of literal creation
(for Objects, Arrays, Regexps, and at some point Classes).

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1472323002

Cr-Commit-Position: refs/heads/master@{#32273}