Rename and rework `_LIBCPP_TRIVIAL_PAIR_COPY_CTOR`. Move FreeBSD configuration in-tree.

This patch does the following:

* It renames `_LIBCPP_TRIVIAL_PAIR_COPY_CTOR` to `_LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR`.
* It automatically enables this option on FreeBSD in ABI V1, since that's the current ABI FreeBSD ships.
* It cleans up the handling of this option in `std::pair`.

I would like the sign off from the FreeBSD maintainers. They will no longer need to keep their `__config` changes downstream.

I'm still hoping to come up with a better way to maintain the ABI without needing these constructors.

Reviewed in https://reviews.llvm.org/D21329

llvm-svn: 275749
diff --git a/libcxx/include/utility b/libcxx/include/utility
index 9b8e792..7b978ad 100644
--- a/libcxx/include/utility
+++ b/libcxx/include/utility
@@ -285,9 +285,6 @@
     _T1 first;
     _T2 second;
 
-    // pair(const pair&) = default;
-    // pair(pair&&) = default;
-
 #ifndef _LIBCPP_HAS_NO_DEFAULT_FUNCTION_TEMPLATE_ARGS
     template <bool _Dummy = true, class = typename enable_if<
         __dependent_type<is_default_constructible<_T1>, _Dummy>::value &&
@@ -310,10 +307,7 @@
                                       )
             : first(__p.first), second(__p.second) {}
 
-#if !defined(_LIBCPP_HAS_NO_DEFAULTED_FUNCTIONS) && _LIBCPP_TRIVIAL_PAIR_COPY_CTOR
-    _LIBCPP_INLINE_VISIBILITY
-    pair(const pair& __p) = default;
-#elif !defined(_LIBCPP_HAS_NO_RVALUE_REFERENCES) || !_LIBCPP_TRIVIAL_PAIR_COPY_CTOR
+#if defined(_LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR)
     _LIBCPP_INLINE_VISIBILITY
     pair(const pair& __p)
         _NOEXCEPT_(is_nothrow_copy_constructible<first_type>::value &&
@@ -322,6 +316,21 @@
           second(__p.second)
     {
     }
+
+# ifndef _LIBCPP_CXX03_LANG
+    _LIBCPP_INLINE_VISIBILITY
+    pair(pair&& __p) _NOEXCEPT_(is_nothrow_move_constructible<first_type>::value &&
+                                is_nothrow_move_constructible<second_type>::value)
+        : first(_VSTD::forward<first_type>(__p.first)),
+          second(_VSTD::forward<second_type>(__p.second))
+    {
+    }
+# endif
+#elif !defined(_LIBCPP_CXX03_LANG)
+    pair(pair const&) = default;
+    pair(pair&&) = default;
+#else
+  // Use the implicitly declared copy constructor in C++03
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
@@ -353,19 +362,6 @@
             : first(_VSTD::forward<_U1>(__p.first)),
               second(_VSTD::forward<_U2>(__p.second)) {}
 
-#ifndef _LIBCPP_HAS_NO_DEFAULTED_FUNCTIONS
-    _LIBCPP_INLINE_VISIBILITY
-    pair(pair&& __p) = default;
-#else
-    _LIBCPP_INLINE_VISIBILITY
-    pair(pair&& __p) _NOEXCEPT_(is_nothrow_move_constructible<first_type>::value &&
-                                is_nothrow_move_constructible<second_type>::value)
-        : first(_VSTD::forward<first_type>(__p.first)),
-          second(_VSTD::forward<second_type>(__p.second))
-    {
-    }
-#endif
-
     _LIBCPP_INLINE_VISIBILITY
     pair&
     operator=(pair&& __p) _NOEXCEPT_(is_nothrow_move_assignable<first_type>::value &&