[flang][acc] Avoid assert for assumed-size arrays without a descriptor (#205726)
Example:
```fortran
subroutine sub(a, n)
real(8) :: a(n, *)
integer :: n, i
!$acc data no_create(a)
!$acc parallel loop
do i = 1, n
a(1, i) = 0.0d0
end do
!$acc end data
end subroutine
```
An assumed-size dummy array (e.g. `real(8) :: a(n,*)`) has an unknown
trailing extent and is passed without a descriptor. When the OpenACC
implicit-data pass builds a data clause for such an array,
`generateSeqTyAccBounds` enters the unknown-shape branch but finds no
descriptor (`fir.box`) to recover bounds from, and hits:
assert(false && "array with unknown dimension expected to have
descriptor");
The premise is wrong: an assumed-size array legitimately has no
descriptor and no recoverable bounds.
Fix: return empty bounds instead of asserting. The caller only assigns
bounds when non-empty, so the array is mapped without bounds — the only
correct option when the extent is unknown, and sufficient for
presence-only clauses (`no_create`/`present`).
llvm-project
76ddb39f - [flang][acc] Avoid assert for assumed-size arrays without a descriptor (#205726)
