Enhance MLIRContext and operations with the ability to register diagnostic

handlers and to feed them with errors and warnings produced by the compiler.
Enhance Operation to be able to get its own MLIRContext on demand, simplifying
some clients.  Change the verifier to emit certain errors with the diagnostic
handler.

This is steps towards reworking the verifier and diagnostic propagation but is
itself not particularly useful.  More to come.

PiperOrigin-RevId: 206948643