[flang] Fix function result rewrite for CPTR type

Not all derived type can be taken as abstract result. The CPTR type
should be treated as return by value so to interoperable with C
functions. Fix the function result rewrite for CPTR type, but it
should be generalized for all derived types. The ABI of
interoperability with C for derived type is architecture dependent,
which should be supported later.

Reviewed By: PeteSteinfeld, jeanPerier

Differential Revision: https://reviews.llvm.org/D137548