[mlir][OpenMP] Add custom parser and pretty printer for parallel construct

Reviewers: jdoerfert

Subscribers: yaxunl, guansong, mehdi_amini, rriddle, jpienaar, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, stephenneuendorffer, Joonsoo, grosul1, frgossen, Kayjukh, jurahul, sstefan1, msifontes

Tags: #mlir

Differential Revision: https://reviews.llvm.org/D81264