[mlir][emitc] Add a pointer type

Adds a pointer type to EmitC. The emission of pointers is so far only
possible by using the `emitc.opaque` type

Co-authored-by: Simon Camphausen <simon.camphausen@iml.fraunhofer.de>

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D119337