regex: Enable test on Windows

Fixes #13725
Remove workaround of #13793/#10872

LLVM assertion error has been fixed recently:
http://llvm.org/bugs/show_bug.cgi?id=18993

Fixes #13793
auto merge of #16450 : klutzy/rust/win-fixme-cleanup, r=alexcrichton

First commit enables regex test on Windows. It was not working at some point (#13725), but it works now.

Second commit removes various FIXMEs regarding #13793, since upstream bug has been fixed.