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

This is to prevent a GCC warning (
#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

Author: var-const

libcxx/include/regex

@@ -1459,7 +1459,7 @@ struct __state
     _LIBCPP_INLINE_VISIBILITY
     __state()
         : __do_(0), __first_(nullptr), __current_(nullptr), __last_(nullptr),
-          __node_(nullptr), __flags_() {}
+          __node_(nullptr), __flags_(), __at_first_(false) {}
 };
 
 // __node