Escaped contextual keywords are simply valid identifiers if they do not occur
in the context where they are a keyword. Escape sequences of the form \uNNNN
or \u{NNNNNN} must be consumed as part of the identifier.

If such escaped contextual keywords do occur in a context where they are a
keyword, they are a syntax error. In that case we manually check locally
whether they are escaped.

Bug: v8:6543, v8:6541

Change-Id: I7e1557963883e722310b9078d7d7636ec94aa603
Reviewed-on: https://chromium-review.googlesource.com/c/1473293Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59628}