Make SpirvValue contain an enum for its value

[khyperia]

This is a prerequisite for all sorts of work (panics, printf extension, builtin functions, nested constants, all sorts of fun stuff) - SpirvValueKind is eventually going to have a decent number of cases. However, just for a proof-of-concept, I've added one "extra" case: ConstantPointer. Not only does this work towards my goal of removing the horrible zombie_even_in_user_code hack, it significantly improves error messages, since we're able to report violations of using the constant pointer directly in the compiler, not the linker, and so we have span information.

Before:

  error: dynamic use of address of constant
    |
    = note: Stack:
            main_fs
            Unnamed function ID %343

After:

  error: Cannot use this pointer directly, it must be dereferenced first
     --> examples/shaders/sky-shader/src/lib.rs:184:5
      |
  184 |     f(&2);
      |     ^^^^^

[khyperia]

Because this is such a huge change, it's going to merge conflict with pretty much any compiler change, so I'm going to just merge this now.