[flang][NFC] centralize FreeMemOp generation in IntrinsicCall.cpp

The current intrinsic call lowering contains a lot of repetitive
patterns when it comes to dealing with temporary allocatable
results allocated by the runtime that need to be dereferenced and
for which a clean-up (free) must be scheduled in the StatementContext.

For HLFIR lowering, I will need to deal with the clean-up in a different
way since the results will be "moved" into expression nodes and
the clean-up will be inserted in bufferization after the last hlfir.expr
usage. Centralizing the clean-up code will make that easier, and is
regardless of this motivation a quality improvement.

Some static helpers had to be moved to IntrinsicBuilder method so that
they could call the readAndAddCleanUp code.

Differential Revision: https://reviews.llvm.org/D141669