Fix the MLIR integer attribute parser to be correct in the face of large...

Fix the MLIR integer attribute parser to be correct in the face of large integer attributes, it was previously artificially limited to 64 bits.

Reviewers: rriddle!

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78065