[mlir][linalg] adding operation to access the iteration index of enclosing linalg ops.

The `linalg.index` operation provides access to the iteration indexes of immediately enclosing linalg operations. It takes a dimension `dim` attribute and returns the iteration index in the given dimension. Having `linalg.index` allows us to unify `linalg.generic` and `linalg.indexed_generic` and also enables index access in named operations.

Differential Revision: https://reviews.llvm.org/D100292