Mark shared_ptr::__create_with_control_block as noexcept.

[zoecarver]

Summary

The default constructor for shared_ptr and shared_ptr::__enable_weak_this are both noexcept so, shared_ptr::__create_with_control_block can also be marked noexcept.

libcxx/include/memory

template <class _Dp>
         _Dp* __get_deleter() const _NOEXCEPT
             {return static_cast<_Dp*>(__cntrl_
                     ? const_cast<void *>(__cntrl_->__get_deleter(typeid(_Dp)))
                       : nullptr);}
 #endif  // _LIBCPP_NO_RTTI
 
     template<class _Yp, class _CntrlBlk>
     static shared_ptr<_Tp>
-    __create_with_control_block(_Yp* __p, _CntrlBlk* __cntrl)
+    __create_with_control_block(_Yp* __p, _CntrlBlk* __cntrl) _NOEXCEPT
     {
         shared_ptr<_Tp> __r;
         __r.__ptr_ = __p;
         __r.__cntrl_ = __cntrl;
         __r.__enable_weak_this(__r.__ptr_, __r.__ptr_);
         return __r;
     }