[PATCH] [libc++] Try and prevent evaluation of `is_default_constructible` on tuples default constructor if it is not needed.

[Richard Smith]

================
Comment at: include/type_traits:2649-2652
@@ -2648,1 +2648,6 @@
 
+template <class _Tp, bool>
+struct _LIBCPP_TYPE_VIS_ONLY __dependent_is_default_constructible
+    : public is_default_constructible<_Tp>
+    {};
+
----------------
Is it worth generalizing / simpilfying this:

    template <class _Tp, bool> struct __dependent : public _Tp {};

(Usage: `__dependent<is_default_constructible<_Tp>, Dummy>::value`)

http://reviews.llvm.org/D7569

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