I started this diff trying to remove a little dead code from the C
backend, but ended up finding a bunch of dead code sprinkled all over
the place:

* `packed` handling in the C backend which was made dead by `Legalize`
* Representation of pointers to runtime-known vector indices
* Handling for the `vector_store_elem` AIR instruction (now removed)
* Old tuple handling from when they used the InternPool repr of structs
* Straightforward unused functions
* TODOs in the LLVM backend for features which Zig just does not support