[flang][hlfir] Lower NULL(MOLD) to a variable

HLFIR lowering promotes intrinsic results lowered in memory to
hlfir.expr to underline their read-only aspect once they are created.
NULL(MOLD) should not be promoted to an hlfir.expr, it is the NULL
variable (we need to see it as an address).

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D149053