[mlir] Support optional attributes in assembly formats

Summary: This revision adds support for assembly formats with optional attributes. It elides optional attributes that are part of the syntax from the attribute dictionary.

Reviewers: ftynse, Kayjukh

Reviewed By: ftynse, Kayjukh

Subscribers: mehdi_amini, rriddle, jpienaar, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, stephenneuendorffer, Joonsoo, grosul1, frgossen, jurahul, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80113