[flang] Better error message for NULL() actual argument for dummy allocatable

[flang] Better error message for NULL() actual argument for dummy allocatable

f18 intentionally does not support the spottily-implemented language extension
in which one can pass NULL() for an allocatable dummy argument.  This is perhaps
a sanctioned side effect in other compilers of the fact that they pass distinct
"base address" and "descriptor address" physical arguments.

Make the error message in this case more specific to the circumstances, and
add a note to Extensions.md to clarify that this behavior is intended.

(We could, with some effort in lowering, support passing NULL for an INTENT(IN)
allocatable dummy, but let's see whether such nonconforming usage appears
in a real application before spending any more time on it.)

Differential Revision: https://reviews.llvm.org/D130380