[flang] Extension: accept NULL([MOLD=]) for ALLOCATABLE INTENT(IN) du… (#66256)

…mmy argument

Several compilers accept a null pointer (with or without a MOLD=) as an
actual argument for association with an INTENT(IN) allocatable dummy
argument. At runtime, the allocatable dummy argument appears to be in
the unallocated state. This seems useful, unambiguous, unlikely to
invalidate conforming code, and works with Intel, NAG, & XLF, so it
should be supported with an optional portability warning in this
compiler as well.