Fix bug #20335 - memory leak when move-constructing a string with unequal allocator. Thanks to Thomas Koeppe for the report

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@213269 91177308-0d34-0410-b5e6-96231b3b80d8

test/strings/basic.string/string.cons/move_alloc.pass.cpp

@@ -45,6 +45,16 @@ int main()
     test(S("1"), A(5));
     test(S("1234567890123456789012345678901234567890123456789012345678901234567890"), A(7));
     }
+
+    int alloc_count = test_alloc_base::alloc_count;
+    {
+    typedef test_allocator<char> A;
+    typedef std::basic_string<char, std::char_traits<char>, A> S;
+    S s1 ( "Twas brillig, and the slivy toves did gyre and gymbal in the wabe" );
+    S s2 (std::move(s1), A(1));
+    }
+    assert ( test_alloc_base::alloc_count == alloc_count );
+    
 #if __cplusplus >= 201103L
     {
     typedef min_allocator<char> A;