[PATCH] [libcxx] Add <experimental/tuple> header for LFTS.

[Marshall Clow]

================
Comment at: include/experimental/tuple:51
@@ +50,3 @@
+
+#define __cpp_lib_experimental_apply
+
----------------
cppreference.com says: 

    a value of at least `201402` indicates that the tuple apply() function is supported

http://reviews.llvm.org/D4512

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