[libc++] Initialize all member variables of `__state` in `<regex>`

This is to prevent a GCC warning (
https://github.com/llvm/llvm-project/issues/65132). It looks like
`__at_first` is always assigned before it's used, but all other member
variables of this struct are initialized in the constructor, so there is
no reason not to initialize `__at_first` as well.

Differential Revision: https://reviews.llvm.org/D159249