[flang][hlfir] Add an operation for the PACK intrinsic

PACK is listed in flang/docs/HighLevelFIR.md as a planned transformational intrinsic, but is not implemented currently. Lowering always falls through to IntrinsicCall.cpp::genPack() and emits _FortranAPack.

Current behaviour:
```
subroutine pack_true_contig(a, r)
real :: a(10)
real :: r(10)
r = pack(a, .true.)
end subroutine
```

There is no check for compile-time .TRUE. mask. The existing lowering test explicitly expects the runtime call:
`fir.call @_FortranAPack`