[Flang][MLIR][OpenMP] Add support for simdlen clause

This supports lowering from parse-tree to MLIR and translation from
MLIR to LLVM IR using OMPIRBuilder for OpenMP simdlen clause in SIMD
construct.

Reviewed By: shraiysh, peixin, arnamoy10

Differential Revision: https://reviews.llvm.org/D130195