Fix ctRegex unstable parser

I got this error using `dub test` on libasync and it traced back to something complex inside `ctRegex`.

https://gist.github.com/etcimon/c659ef8050b698303614
Note that there is a difference between `auto re = regex("foobar")` and `enum re = regex("foobar")`.
For the latter the regex is still parsed at compile time, maybe even IR generation is done.