[mlir][matchers] Add m_Op(StringRef) and m_Attr matchers

This patch introduces support for m_Op with a StringRef argument and m_Attr matchers. These matchers will be very useful for mlir-query that is being developed currently.

Submitting this patch separately to reduce the final patch size and make it easier to upstream mlir-query.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D147262