[flang] fix C_PTR function result lowering (#100082)
Functions returning C_PTR were lowered to function returning intptr (i64
on 64bit arch). This caused conflicts when these functions were defined
as returning !fir.ref<none>/llvm.ptr in other compiler generated
contexts (e.g., malloc).
Lower them to return !fir.ref<none>.
This should deal with https://github.com/llvm/llvm-project/issues/97325
and https://github.com/llvm/llvm-project/issues/98644.
