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/copy.pass.cpp

@@ -87,6 +87,11 @@ int main()
         optional<T> rhs(3);
         test(rhs);
     }
+    {
+        typedef const int T;
+        optional<T> rhs(3);
+        test(rhs);
+    }
     {
         typedef X T;
         optional<T> rhs;
@@ -97,6 +102,11 @@ int main()
         optional<T> rhs(X(3));
         test(rhs);
     }
+    {
+        typedef const X T;
+        optional<T> rhs(X(3));
+        test(rhs);
+    }
     {
         typedef Y T;
         optional<T> rhs;