[mlir] Add initial support for parsing a declarative operation assembly format

Summary:
This is the first revision in a series that adds support for declaratively specifying the asm format of an operation. This revision
focuses solely on parsing the format. Future revisions will add support for generating the proper parser/printer, as well as
transitioning the syntax definition of many existing operations.

This was originally proposed here:
https://llvm.discourse.group/t/rfc-declarative-op-assembly-format/340

Differential Revision: https://reviews.llvm.org/D73405