ParserBase::is_any_identifier currently does not recognise
Token::ESCAPED_STRICT_RESERVED_WORD as an identifier. This seems different
from what ParserBase::ParseIdentifierName does, and also prevents
"l\u0065t", unlike "let", from becoming a label.

This CL extends is_any_identifier to also accept ESCAPED_STRICT_RESERVED_WORD.

BUG=v8:5692

Review-Url: https://codereview.chromium.org/2695973003
Cr-Commit-Position: refs/heads/master@{#43204}