[mlir][llvm] Allow literal structs to replaceImmediateSubElements

SubElementInterfaces forbids all mutable types and attributes from
implementing `replaceImmediateSubElements`. However, this prohibits
literal structs, which are immutable, from implementing that function.
This patch defers the decision on whether to support
`replaceImmediateSubElements` to the individual types/attributes.

Depends on D136505

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D136507