Make the end position of a regexp literal the first character following the regexp. This matches the behaviour of number literals and string literals, as well as single-character tokens.

This change corrects the lazy-parsing of arrow functions with concise bodies, whose last token is a regular expression literal.

BUG=v8:4474
LOG=N
R=wingo@igalia.com, adamk@chromium.org, rossberg@chromium.org

Review URL: https://codereview.chromium.org/1389313003

Cr-Commit-Position: refs/heads/master@{#31189}