Add <optional> header.

This patch is largely thanks to Casey Carter @ Microsoft. He did the initial
work of porting our experimental implementation and tests over to namespace
std.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@283977 91177308-0d34-0410-b5e6-96231b3b80d8

src/optional.cpp

@@ -7,18 +7,18 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "optional"
 #include "experimental/optional"
 
-_LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL
-
-#ifdef _LIBCPP_HAS_NO_DEFAULTED_FUNCTIONS
-
-bad_optional_access::~bad_optional_access() _NOEXCEPT {}
-
-#else
+namespace std
+{
 
 bad_optional_access::~bad_optional_access() _NOEXCEPT = default;
 
-#endif
+} // std
+
+_LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL
+
+bad_optional_access::~bad_optional_access() _NOEXCEPT = default;
 
 _LIBCPP_END_NAMESPACE_EXPERIMENTAL