[mlir][bufferization] Use Type instead of Value in unknown conversion (#144658)

Generally, bufferization should be able to create a memref from a tensor
without needing to know more than just a mlir::Type. Thus, change
BufferizationOptions::UnknownTypeConverterFn to accept just a type
(mlir::TensorType for now) instead of mlir::Value. Additionally, apply
the same rationale to getMemRefType() helper function.

Both changes are prerequisites to enable custom types support in
one-shot bufferization.

GitOrigin-RevId: a1c2a712939897251729b6fc436a2db7db6f03fc