Fix a bug in the .mlir lexer, where a \0 character in a file is treated as a...

Fix a bug in the .mlir lexer, where a \0 character in a file is treated as a colon (due to an accidental fall through) instead of whitespace.

Summary:
While here, simplify the lexer a bit by eliminating the unneeded 'operator'
classification of certain sigils, they can just be treated as 'punctuation'.

Reviewers: rriddle!

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76647