[HLSL] Add semantic signature packing command line flags

DXC provides two flags to specify the kind of packing that is used when packing signature elements: `-pack-prefix-stable` and `-pack-optimized`.

This issue tracks adding these to the DXC driver mode and as a dx flag in clang.

These should then be used in `emitEntryFunction` to determine how signature packing is invoked, defined here: https://github.com/llvm/llvm-project/issues/57928.

AC:
- [ ] `-pack-prefix-stable` and `-pack-optimized` are DXC driver flags
- [ ] `-fdx-semantic-signature-packing-mode=` is a clang cmd line option
- [ ] Add lit tests to ensure the flags change (or don't) the packing behaviour as expected