[mlir][OpenMP] Add nontemporal clause definition to simd construct

simd nontemporal construct is represented as a list of variables
which have low locality accross simd iterations

Added verifier of nontemporal clause. MLIR tests were updated to test
correctness of MLIR definition of nontemporal clause.

Differential Revision: https://reviews.llvm.org/D140553

Reviewed By: kiranchandramohan