[mlir] Promote the SubElementInterfaces to a core Attribute/Type construct

This commit restructures the sub element infrastructure to be a core part
of attributes and types, instead of being relegated to an interface. This
establishes sub element walking/replacement as something "always there",
which makes it easier to rely on for correctness/etc (which various bits of
infrastructure want, such as Symbols).

Attribute/Type now have `walk` and `replace` methods directly
accessible, which provide power API for interacting with sub elements. As
part of this, a new AttrTypeWalker class is introduced that supports caching
walked attributes/types, and a friendlier API (see the simplification of symbol
walking in SymbolTable.cpp).

Differential Revision: https://reviews.llvm.org/D142272