[flang] Improve handling of NULL() arguments to intrinsics (#93866)

Some intrinsics (extends_type_of, same_type_as) can accept a NULL actual
argument so long as it has a MOLD=. Some intrinsics that are marked in
the intrinsics table as accepting a NULL actual argument already should
only do so if it has a MOLD=. Distinguish table entries that accept a
NULL() only with a MOLD= from the few others that allow a bare NULL()
and update tests.

Fixes https://github.com/llvm/llvm-project/issues/93845.