[mlir][llvm] Move LLVMArrayType to a TypeDef

This moves the `LLVMArrayType` to a `TypeDef`. The main side-effect of
this change is that the syntax `array<4xi32>` is no longer allowed. It
was previously parsed and then printed as `array<4 x i32>`. Now the
syntax must be the latter.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D136473