Unify type uniquing and construction.

This allows for us to decouple type uniquing/construction from MLIRContext and pave the way for dialect specific types.

To accomplish this we two new classes, TypeUniquer and TypeStorageAllocator.

* TypeUniquer is now responsible for all construction and uniquing of types.
* TypeStorageAllocator is a utility used by derived type storage objects to allocate memory within an MLIRContext.

This cl also standardizes what a derived type storage class needs to provide:
    - Define a type alias, KeyTy, to a type that uniquely identifies the
      instance of the type within its kind.
      * The key type must be constructible from the values passed into the
        detail::TypeUniquer::get call after the type kind.
      * The key type must have a llvm::DenseMapInfo specialization for
        hashing.

    - Provide a method, 'KeyTy getKey() const', to construct the key type
      from an existing storage instance.

    - Provide a construction method:
        'DerivedStorage *construct(TypeStorageAllocator &, ...)'
      that builds a unique instance of the derived storage. The arguments
      after the TypeStorageAllocator must correspond with the values passed
      into the detail::TypeUniquer::get call after the type kind.

PiperOrigin-RevId: 226507184