[MLIR] Deduplicate dialect registration by ClassID

Summary:
With the move towards dialect registration that does not depend only use
static initialization, we are running into more cases where the dialects
are registered by different methods. For example, TensorFlow still uses
static initialization to register all MLIR core dialects, which prevents
explicit registration of any of them when linking it in. We ran into this
issue in https://github.com/google/iree/pull/982.

To address potential issues with conflicts from non-standard
allocators passed to registerDialectAllocator, made this method
private. Now all dialects can only be registered with their
constructor.

Similarly deduplicates DialectHooks for consistency and makes their
registration follow the same pattern.

Differential Revision: https://reviews.llvm.org/D76329