Refactor default constructor SFINAE in pair and tuple.

Refactor the  recent implicit default constructor changes to match the
existing SFINAE style.

llvm-svn: 373263
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: c8ad8686ac072fb35862f4d5e09cb1ab91a15a8e
diff --git a/include/__tuple b/include/__tuple
index 196f3c2..4da9ec5 100644
--- a/include/__tuple
+++ b/include/__tuple
@@ -477,8 +477,9 @@
                               >;
 
 struct _LIBCPP_TYPE_VIS __check_tuple_constructor_fail {
-    template <class ...>
-    static constexpr bool __enable_default() { return false; }
+
+    static constexpr bool __enable_explicit_default() { return false; }
+    static constexpr bool __enable_implicit_default() { return false; }
     template <class ...>
     static constexpr bool __enable_explicit() { return false; }
     template <class ...>