[mlir] add an interface to support custom types in LLVM dialect pointers

This may be necessary in partial multi-stage conversion when a container type
from dialect A containing types from dialect B goes through the conversion
where only dialect A is converted to the LLVM dialect. We will need to keep a
pointer-to-non-LLVM type in the IR until a further conversion can convert
dialect B types to LLVM types.

Reviewed By: wsmoses

Differential Revision: https://reviews.llvm.org/D106076