EscapeRegExpPattern: references to Pattern omit setting for N

[jmdyck]

In the abstract operation EscapeRegExpPattern, step 1 says

Let _S_ be a String in the form of a |Pattern[~U]| (|Pattern[+U]| if _F_ contains `"u"`)  ...

However, since "RegExp named capture groups" was merged (95ec0c6), Pattern now takes an additional grammatical parameter, N. Thus, the above references to Pattern must be updated, and possibly the logic extended to decide between ~N and +N.

(attn @mathiasbynens)