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/assign_value.pass.cpp

@@ -19,6 +19,14 @@
 
 using std::experimental::optional;
 
+struct AllowConstAssign {
+  AllowConstAssign() = default;
+  AllowConstAssign(AllowConstAssign const&) {}
+  AllowConstAssign const& operator=(AllowConstAssign const&) const {
+      return *this;
+  }
+};
+
 struct X
 {
 };
@@ -52,6 +60,11 @@ int main()
         assert(static_cast<bool>(opt) == true);
         assert(*opt == i);
     }
+    {
+        optional<const AllowConstAssign> opt;
+        const AllowConstAssign other;
+        opt = other;
+    }
     {
         optional<std::unique_ptr<int>> opt;
         opt = std::unique_ptr<int>(new int(3));