[tuple.syn] fix ignore-type::operator= return type

[Tsche]

ignore-type's operator= is currently specified as

constexpr const ignore-type operator=(const auto &) const noexcept { return *this; }

this should instead return const ignore-type&.

@eisenwave noted P2968R2 specifies ignore-type as

+// ignore
+struct ignore-type { // exposition only
+  constexpr const ignore-type&
+    operator=(const auto &) const noexcept
+      { return *this; }
+};
+inline constexpr ignore-type ignore;
// 22.4.5, tuple creation functions
-inline constexpr unspecified ignore;

however, #7109 drops the reference qualification on ignore-type::operator='s return type. Therefore I believe this is an editorial issue.

[jensmaurer]

@jwakely , do you agree?

[jwakely]

I agree