[flang][openacc] Add OpenACC pointer interface to FIR pointer types

The OpenACC dialect data clauses require for variables appearing in data
operations to implement the `PointerLikeType` interface (similarly to
OpenMP dialect). Thus, this interface needs attached to FIR types.

Two of the FIR types, HeapType and LLVMPointerType, did not provide
getElementType - now they do by calling getEleTy. This cleans up the
interface attachment for both OpenACC and OpenMP dialects.

Reviewed By: clementval, jeanPerier

Differential Revision: https://reviews.llvm.org/D148560