[mlir] Add IteratorType enum to StructuredOpsUtils.

[mlir] Add IteratorType enum to StructuredOpsUtils.

Summary:
Use the new enum in TilingIterface and verify that `iterator_type` attribute in
LinalgOp interface is compatible with the enum values. Later IteratorType enum
will be used in LinalgInterface to replace the current `iterator_type` attribute
array of string.

Existing enums in Linalg are moved into a separate td file and tablegen build
target. This is necessary, have one I32EnumAttr in a shared space that generated
enum class definition and EnumAttrs is dialect-specific location. Otherwise
there might be a conflict that I32EnumAttr generates enum definitions in
multiple places.

Differential Revision: https://reviews.llvm.org/D134634