[MLIR] LLVM dialect: modernize and cleanups

Summary:
Modernize some of the existing custom parsing code in the LLVM dialect.
While this reduces some boilerplate code, it also reduces the precision
of the diagnostic error messges.

Reviewers: ftynse, nicolasvasilache, rriddle

Reviewed By: rriddle

Subscribers: merge_guards_bot, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, arpith-jacob, mgester, lucyrfox, liufengdb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72967