Fix PR30260 - optional<const T> not working.

This patch fixes PR30260 by using a (void*) cast on the placement argument
to placement new to casts away the const. See also http://llvm.org/PR30260.

As a drive by change this patch also changes the header guard for
<experimental/optional> to _LIBCPP_EXPERIMENTAL_OPTIONAL from _LIBCPP_OPTIONAL.

llvm-svn: 280775

libcxx/test/std/experimental/optional/optional.object/optional.object.ctor/move.pass.cpp

@@ -74,6 +74,17 @@ public:
     friend constexpr bool operator==(const Z& x, const Z& y) {return x.i_ == y.i_;}
 };
 
+
+class ConstMovable
+{
+    int i_;
+public:
+    ConstMovable(int i) : i_(i) {}
+    ConstMovable(const ConstMovable&& x) : i_(x.i_) {}
+    ~ConstMovable() {i_ = 0;}
+    friend bool operator==(const ConstMovable& x, const ConstMovable& y) {return x.i_ == y.i_;}
+};
+
 int main()
 {
     {
@@ -86,6 +97,11 @@ int main()
         optional<T> rhs(3);
         test(rhs);
     }
+    {
+        typedef const int T;
+        optional<T> rhs(3);
+        test(rhs);
+    }
     {
         typedef X T;
         optional<T> rhs;
@@ -96,6 +112,11 @@ int main()
         optional<T> rhs(X(3));
         test(rhs);
     }
+    {
+        typedef const ConstMovable T;
+        optional<T> rhs(ConstMovable(3));
+        test(rhs);
+    }
     {
         typedef Y T;
         optional<T> rhs;