[flang] Handle optional TARGET associate in ASSOCIATED runtime

The TARGET argument of ASSOCIATED may be dynamically optional, in which
case ASSOCIATED(POINTER, TARGET) is equal to ASSOCIATED(TARGET).

Make the runtime argument a pointer so that it can detect and handle
arguments that are dynamically optional.

Also fix the runtime to check if TARGET base address is not null and if
its element size is not null to match the requirement of ASSOCIATED
regarding TARGET:
- if TARGET is an object: true iff [..] TARGET is not a zerosized storage sequence
- if TARGET is a POINTER: true iff [..] POINTER and TARGET are associated

Not that ASSOCIATED will also returns false if TARGET is an unallocated allocatable.
This is not described in the standard, but is a unanimous behaviour of
existing compilers.

Differential Revision: https://reviews.llvm.org/D120835