Decouple registring passes from specifying argument/description

This patch changes the (not recommended) static registration API from:

 static PassRegistration<MyPass> reg("my-pass", "My Pass Description.");

to:

 static PassRegistration<MyPass> reg;

And the explicit registration from:

  void registerPass("my-pass", "My Pass Description.",
                    [] { return createMyPass(); });

To:

  void registerPass([] { return createMyPass(); });

It is expected that Pass implementations overrides the getArgument() method
instead. This will ensure that pipeline description can be printed and parsed
back.

Differential Revision: https://reviews.llvm.org/D104421