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

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

simd aligned construct is represented as pair of variable which needs
to be aligned and corresponding alignment value.

Added parser, printer and verifier of aligned clause. MLIR tests were
updated to test correctness of MLIR definition of aligned clause.

Differential Revision: https://reviews.llvm.org/D135865

Reviewed By: kiranchandramohan