A previous patch lifting the restriction on invalid escape sequences in
tagged templates had a bug when two template tokens appeared immediately
adject to each other. This moves invalid escape information from the
tokenizer state proper into the TokenDesc, preventing the overwriting
which caused this issue.

Previous CL is at
https://codereview.chromium.org/2665513002

BUG=v8:6029,v8:5546

Review-Url: https://codereview.chromium.org/2724003006
Cr-Commit-Position: refs/heads/master@{#43596}