[flang] Remove legacy (non-HLFIR) lowering and related options (#196205)
Drop the user-facing options that selected the legacy non-HLFIR lowering
path, the always-true `LowerToHighLevelFIR` lowering option, and the
descriptor-discretization debug switch:
- `-flang-experimental-hlfir` and `-flang-deprecated-no-hlfir` (flang
driver and `-fc1`)
- `-hlfir` / `--hlfir` (bbc)
- `--use-desc-for-alloc` (bbc, debug)
Remove every `if (lowerToHighLevelFIR()) { ... } else { ... }` branch in
`lib/Lower/`, keeping the HLFIR side. Delete the now-unused legacy
helpers in `Bridge.cpp` (`copyVarFIR`, both
`genNoHLFIRPointerAssignment`
overloads, the legacy block of `genAssignment`) and the always-empty
`createMutableProperties` together with its four dead helpers in
`Allocatable.cpp`. Drop the corresponding `alwaysUseBox` parameter from
`createMutableBox`.
Note: tests were modified in
https://github.com/llvm/llvm-project/pull/196137
Assisted-by: AI
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