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

test/std/strings/basic.string/string.modifiers/string_assign/string.pass.cpp

@@ -76,4 +76,10 @@ int main()
          S("12345678901234567890"));
     }
 #endif
+#if __cplusplus > 201402L
+    {
+    typedef std::string S;
+    static_assert(noexcept(S().assign(S())), "");  // LWG#2063
+    }
+#endif
 }