[mlir] Prevent SubElementInterface from going into infinite recursion

Since only mutable types and attributes can go into infinite recursion
inside SubElementInterface::walkSubElement, and there are only a few of
them (mutable types and attributes), we introduce new traits for Type
and Attribute: TypeTrait::IsMutable and AttributeTrait::IsMutable,
respectively. They indicate whether a type or attribute is mutable.
Such traits are required if the ImplType defines a `mutate` function.

Then, inside SubElementInterface, we use a set to record visited mutable
types and attributes that have been visited before.

Differential Revision: https://reviews.llvm.org/D127537