Sync regex code with Tcl 8.5.11.

Sync our regex code with upstream changes since last time we did this,
which was Tcl 8.5.0 (see commit df1e965e).

There are no functional changes here; the main point is just to lay down
a commit-log marker that somebody has looked at this recently, and to do
what we can to keep the two codebases comparable.