* LLVM backend: The `alloc` AIR instruction as well as pointer
   constants which point to a 0-bit element type now call a common
   codepath to produce a `*const llvm.Value` which is a non-zero pointer
   with a bogus-but-properly-aligned address.
 * LLVM backend: improve the lowering of optional types.
 * Type: `hasCodeGenBits()` now returns `true` for pointers even when
   it returns `false` for their element types.

Effectively, #6706 is now implemented in stage2 but not stage1.