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

[var-const]

Summary

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.

[ldionne]

LGTM w/ green CI.

libcxx/include/regex

1462

Typo!

[var-const]

Fix typo

libcxx/include/regex

struct __state
     vector<pair<size_t, const _CharT*> > __loop_data_;
     const __node<_CharT>* __node_;
     regex_constants::match_flag_type __flags_;
     bool __at_first_;
 
     _LIBCPP_INLINE_VISIBILITY
     __state()
         : __do_(0), __first_(nullptr), __current_(nullptr), __last_(nullptr),
-          __node_(nullptr), __flags_() {}
+          __node_(nullptr), __flags_(), __at_first_(false) {}
 };
 
 // __node
 
 template <class _CharT>
 class __node
 {
     __node(const __node&);