tuple was accidentally lacking a valid copy assignment operator. It went undetected because I had failed to test assigning from a const lvalue. This fixes http://llvm.org/bugs/show_bug.cgi?id=11921

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@150613 91177308-0d34-0410-b5e6-96231b3b80d8

test/utilities/tuple/tuple.tuple/tuple.assign/copy.pass.cpp

@@ -42,7 +42,7 @@ int main()
     }
     {
         typedef std::tuple<int, char, std::string> T;
-        T t0(2, 'a', "some text");
+        const T t0(2, 'a', "some text");
         T t;
         t = t0;
         assert(std::get<0>(t) == 2);