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.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@280775 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/experimental/optional/optional.object/optional.object.assign/emplace.pass.cpp

@@ -80,6 +80,12 @@ int main()
         assert(static_cast<bool>(opt) == true);
         assert(*opt == 1);
     }
+    {
+        optional<const int> opt(2);
+        opt.emplace(1);
+        assert(static_cast<bool>(opt) == true);
+        assert(*opt == 1);
+    }
     {
         optional<X> opt;
         opt.emplace();