Remove DialectHooks and introduce a Dialect Interfaces instead

These hooks were introduced before the Interfaces mechanism was available.

DialectExtractElementHook is unused and entirely removed. The
DialectConstantFoldHook is used a fallback in the
operation fold() method, and is replaced by a DialectInterface.
The DialectConstantDecodeHook is used for interpreting OpaqueAttribute
and should be revamped, but is replaced with an interface in 1:1 fashion
for now.

Differential Revision: https://reviews.llvm.org/D85595