Implement LWG#2063, and update the issues links to point to the github generated pages

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@249325 91177308-0d34-0410-b5e6-96231b3b80d8

include/string

@@ -1509,6 +1509,7 @@ public:
 #ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
     _LIBCPP_INLINE_VISIBILITY
     basic_string& assign(basic_string&& str)
+        _NOEXCEPT_((__noexcept_move_assign_container<_Allocator, __alloc_traits>::value))
         {*this = _VSTD::move(str); return *this;}
 #endif
     basic_string& assign(const basic_string& __str, size_type __pos, size_type __n=npos);