[mlir] Implement replacement of SymbolRefAttrs in Dialect attributes using SubElementAttr interface

This patch extends the SubElementAttr interface to allow replacing a contained sub attribute. The attribute that should be replaced is identified by an index which denotes the n-th element returned by the accompanying walkImmediateSubElements method.

Using this addition the patch implements replacing SymbolRefAttrs contained within any dialect attributes.

Differential Revision: https://reviews.llvm.org/D111357