Add deduction guides for optional

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@333251 91177308-0d34-0410-b5e6-96231b3b80d8

include/optional

@@ -139,6 +139,10 @@ namespace std {
   private:
     T *val; // exposition only
   };
+
+template<class T>
+  optional(T) -> optional<T>;
+
 } // namespace std
 
 */
@@ -1003,6 +1007,11 @@ private:
     }
 };
 
+#ifndef _LIBCPP_HAS_NO_DEDUCTION_GUIDES
+template<class T>
+    optional(T) -> optional<T>;
+#endif
+
 // Comparisons between optionals
 template <class _Tp, class _Up>
 _LIBCPP_INLINE_VISIBILITY constexpr