- Rely more heavily on Token::IsValidIdentifier.
- Deal with IsLet() when it's possibly a lexical declaration.
- Remove ENUM from the default IsAnyIdentifier range.
- Always pre-check whether IsAnyIdentifier before classifying identifiers.

Change-Id: I55eae6ff65dc306b466fa29d233c715e85bc3854
Reviewed-on: https://chromium-review.googlesource.com/c/1356514Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57977}