[flang] Require contiguous actual pointer for contiguous dummy pointer (#139298)
When the actual argument associated with an explicitly CONTIGUOUS
pointer dummy argument is itself a pointer, it must also be contiguous.
(A non-pointer actual argument can associate with a CONTIGUOUS pointer
dummy argument if it's INTENT(IN), and in that case it's still just a
warning if we can't prove at compilation time that the actual is
contiguous.)
Fixes https://github.com/llvm/llvm-project/issues/138899.