[flang] system_clock intrinsic calls with dynamically optional arguments

[flang] system_clock intrinsic calls with dynamically optional arguments

system_clock intrinsic calls with dynamically optional arguments

Modify intrinsic system_clock calls to allow for an argument that is optional
or a disassociated pointer or an unallocated allocatable.  A call with such an
argument is the same as a call that does not specify that argument.

Rename (genIsNotNull -> genIsNotNullAddr) and (genIsNull -> genIsNullAddr)
and add a use of genIsNotNullAddr.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D127616

Co-authored-by: V Donaldson <vdonaldson@nvidia.com>