[mlir] Add a new builtin `unrealized_conversion_cast` operation

An `unrealized_conversion_cast` operation represents an unrealized conversion
from one set of types to another, that is used to enable the inter-mixing of
different type systems. This operation should not be attributed any special
representational or execution semantics, and is generally only intended to be
used to satisfy the temporary intermixing of type systems during the conversion
of one type system to another.

This operation was discussed in the following RFC(and ODM):

https://llvm.discourse.group/t/open-meeting-1-14-dialect-conversion-and-type-conversion-the-question-of-cast-operations/

Differential Revision: https://reviews.llvm.org/D94832