Sync regex code with Tcl 8.5.11.

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 df1e965e12cdd48c11057ee6e15346ee2b8b02f5).

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.