Make TcUnionPieceView movable to satisfy std::ranges::viewable_range (#9567)

Summary:
Building type-constraint.h with clang 18 + libc++ 18 currently fails, because TcUnionPieceView isn't movable:

```
In file included from /hhvm/hphp/system/systemlib.cpp:19:
In file included from /hhvm/hphp/runtime/base/array-init.h:21:
In file included from /hhvm/hphp/runtime/base/array-data-defs.h:23:
In file included from /hhvm/hphp/runtime/base/type-variant.h:20:
In file included from /hhvm/hphp/runtime/base/tv-conversions.h:23:
In file included from /hhvm/hphp/runtime/base/type-object.h:19:
In file included from /hhvm/hphp/runtime/base/object-data.h:27:
In file included from /hhvm/hphp/runtime/vm/class.h:32:
In file included from /hhvm/hphp/runtime/vm/preclass.h:28:
/hhvm/hphp/runtime/vm/type-constraint.h:996:15: error: static assertion failed
  996 | static_assert(std::ranges::viewable_range<HPHP::TcUnionPieceView>);
      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/hhvm/hphp/runtime/vm/type-constraint.h:996:15: note: because 'HPHP::TcUnionPieceView' does not satisfy 'viewable_range'
/usr/bin/../include/c++/v1/__ranges/concepts.h:131:7: note: because 'remove_cvref_t<TcUnionPieceView>' (aka 'HPHP::TcUnionPieceView') does not satisfy 'view'
  131 |     ((view<remove_cvref_t<_Tp>> && constructible_from<remove_cvref_t<_Tp>, _Tp>) ||
      |       ^
/usr/bin/../include/c++/v1/__ranges/concepts.h:94:30: note: because 'HPHP::TcUnionPieceView' does not satisfy 'movable'
   94 | concept view = range<_Tp> && movable<_Tp> && enable_view<_Tp>;
      |                              ^
/usr/bin/../include/c++/v1/__concepts/movable.h:29:66: note: because 'assignable_from<HPHP::TcUnionPieceView &, HPHP::TcUnionPieceView>' evaluated to false
   29 | concept movable = is_object_v<_Tp> && move_constructible<_Tp> && assignable_from<_Tp&, _Tp> && swappable<_Tp>;
      |                                                                  ^
/usr/bin/../include/c++/v1/__concepts/assignable.h:34:15: note: because '__lhs = std::forward<_Rhs>(__rhs)' would be invalid: object of type 'HPHP::TcUnionPieceView' cannot be assigned because its copy assignment operator is implicitly deleted
   34 |       { __lhs = std::forward<_Rhs>(__rhs) } -> same_as<_Lhs>;
      |               ^
/usr/bin/../include/c++/v1/__ranges/concepts.h:133:8: note: and 'is_lvalue_reference_v<HPHP::TcUnionPieceView>' evaluated to false
  133 |       (is_lvalue_reference_v<_Tp> ||
      |        ^
/usr/bin/../include/c++/v1/__ranges/concepts.h:134:9: note: and 'remove_reference_t<TcUnionPieceView>' (aka 'HPHP::TcUnionPieceView') does not satisfy 'movable'
  134 |        (movable<remove_reference_t<_Tp>> && !__is_std_initializer_list<remove_cvref_t<_Tp>>))));
      |         ^
/usr/bin/../include/c++/v1/__concepts/movable.h:29:66: note: because 'assignable_from<HPHP::TcUnionPieceView &, HPHP::TcUnionPieceView>' evaluated to false
   29 | concept movable = is_object_v<_Tp> && move_constructible<_Tp> && assignable_from<_Tp&, _Tp> && swappable<_Tp>;
      |                                                                  ^
/usr/bin/../include/c++/v1/__concepts/assignable.h:34:15: note: because '__lhs = std::forward<_Rhs>(__rhs)' would be invalid: object of type 'HPHP::TcUnionPieceView' cannot be assigned because its copy assignment operator is implicitly deleted
   34 |       { __lhs = std::forward<_Rhs>(__rhs) } -> same_as<_Lhs>;
```

The same error also occurs with GCC 14.

So, make the members non-const to satisfy std::movable.

Split from #9564.

Pull Request resolved: #9567

Reviewed By: paulbiss

Differential Revision: D67907638

fbshipit-source-id: 11c83335221dbb314e69343c23321f16ee5a045b

Author: mszabo-wikia

hphp/runtime/vm/type-constraint.h

@@ -669,8 +669,8 @@ struct TcUnionPieceView {
   TcUnionPieceIterator begin() const;
   TcUnionPieceIterator end() const;
 private:
-  const TypeConstraint m_tc;
-  const Kind m_kind;
+  TypeConstraint m_tc;
+  Kind m_kind;
 };
 
 static_assert(std::ranges::input_range<TcUnionPieceView>);