Fix PR26961 - Add default constructor to std::pointer_safety struct.

In ABI v1 libc++ implements std::pointer_safety as a class type instead
of an enumeration. However this class type does not provide
a default constructor as it should. This patch adds that default constructor.

llvm-svn: 291059
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 76a01ea34d6574a4333d98eeb3b8bc7f9472f579
diff --git a/include/memory b/include/memory
index 3ea9a77..a38d95b 100644
--- a/include/memory
+++ b/include/memory
@@ -5637,6 +5637,9 @@
     __lx __v_;
 
     _LIBCPP_INLINE_VISIBILITY
+    pointer_safety() : __v_() {}
+
+    _LIBCPP_INLINE_VISIBILITY
     pointer_safety(__lx __v) : __v_(__v) {}
     _LIBCPP_INLINE_VISIBILITY
     operator int() const {return __v_;}