Hide some public API of QuantizedType inherited from Type.

It is for preventing subtle bugs when writing predicates with wrong APIs. For example,

// BAD example for checking a quant type has an i8 or i16 storage type.
auto qtype = type.dyn_cast<mlir::quant::UniformQuantizedType>()
if (qtype.isSigned() && (qtype.isInteger(8) || qtype.isInteger(16)))

PiperOrigin-RevId: 257823263