Poison "quantized int" specializations of std::is_signed, as a first …

…step towards removing them.

This specialization is not permitted per the C++ standard, and thus:
- MSVC silently ignores specializations of `std::is_signed`, since VS2019 16.4.
- Clang emits `error: 'is_signed' cannot be specialized: Users are not allowed to specialize this standard library entity [-Winvalid-specialization]` by default, as of Clang 20.

Also, modify all relevant callers of `std::is_signed<T>` for which the change triggered a compilation failure to instead call `std::numeric_limits<T>::is_signed`.

Note that `std::numeric_limits` _is_ permitted to be specialized for user types.

PiperOrigin-RevId: 719339288

xla/tsl/framework/type_traits.h

@@ -98,17 +98,22 @@ class numeric_limits<tsl::quint16> : public numeric_limits<tsl::uint16> {};
 template <>
 class numeric_limits<tsl::qint32> : public numeric_limits<tsl::int32> {};
 
-// Specialize is_signed for quantized types.
+// Templates from <type_traits> are not permitted to be specialized by users,
+// and doing so may be ignored or cause an error. As a transitional measure,
+// since tensorflow was specializing is_signed for some time, poison uses by
+// specializing with an empty struct (missing the `value` member).
+//
+// TODO(b/392034954): Delete these invalid specializations.
 template <>
-struct is_signed<tsl::qint8> : public is_signed<tsl::int8> {};
+struct is_signed<tsl::qint8> {};
 template <>
-struct is_signed<tsl::quint8> : public is_signed<tsl::uint8> {};
+struct is_signed<tsl::quint8> {};
 template <>
-struct is_signed<tsl::qint16> : public is_signed<tsl::int16> {};
+struct is_signed<tsl::qint16> {};
 template <>
-struct is_signed<tsl::quint16> : public is_signed<tsl::uint16> {};
+struct is_signed<tsl::quint16> {};
 template <>
-struct is_signed<tsl::qint32> : public is_signed<tsl::int32> {};
+struct is_signed<tsl::qint32> {};
 
 }  // namespace std