Add C++17 explicit deduction guides to std::pair.

This patch adds the newly standardized deduction guides
for std::pair, allowing it to work class template deduction.

llvm-svn: 314864
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: af65856eec160d163c764faad250d93357be7c83
diff --git a/include/tuple b/include/tuple
index a52b934..788fd75 100644
--- a/include/tuple
+++ b/include/tuple
@@ -929,7 +929,7 @@
     void swap(tuple&) _NOEXCEPT {}
 };
 
-#ifdef __cpp_deduction_guides
+#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
 // NOTE: These are not yet standardized, but are required to simulate the
 // implicit deduction guide that should be generated had libc++ declared the
 // tuple-like constructors "correctly"