[libc++] Improves UB handling in ios_base destructor. (#76525)

Destroying an ios_base object before it is properly initialized is
undefined behavior. Unlike typical C++ classes the initialization is not
done in the constructor, but in a dedicated init function. Due to
virtual inheritance of the basic_ios object in ostream and friends this
undefined behaviour can be triggered when inheriting from classes that
can throw in their constructor and inheriting from ostream.

Use the __loc_ member of ios_base as sentinel to detect whether the
object has or has not been initialized.

Addresses https://github.com/llvm/llvm-project/issues/57964