[mlir] Change the syntax of dense arrays

Follow-up to D123774, where the syntax of dense arrays was discussed. It
was included that the syntax should be changed to `array<i32: 1, 2>`.
This patch changes the syntax but importantly preserves the `[1, 2]`
syntax when embedding these attributes in assembly formats through ODS.

Reviewed By: mehdi_amini, jpienaar

Differential Revision: https://reviews.llvm.org/D131738