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/copy.pass.cpp

@@ -19,6 +19,13 @@
 
 using std::experimental::optional;
 
+struct AllowConstAssign {
+  AllowConstAssign(AllowConstAssign const&) {}
+  AllowConstAssign const& operator=(AllowConstAssign const&) const {
+      return *this;
+  }
+};
+
 struct X
 {
     static bool throw_now;
@@ -42,6 +49,11 @@ int main()
         static_assert(static_cast<bool>(opt2) == false, "");
         assert(static_cast<bool>(opt) == static_cast<bool>(opt2));
     }
+    {
+        optional<const AllowConstAssign> opt;
+        optional<const AllowConstAssign> opt2;
+        opt = opt2;
+    }
     {
         optional<int> opt;
         constexpr optional<int> opt2(2);