Sema: prevent comptime-mutable pointers leaking into runtime code

[mlugg]

Resolves: #10920

[mlugg]

i don't really get why this is failing, the code that's erroring shouldn't have anything comptime-mutable whatsoever. will look at it tomorrow

[mlugg]

This is blocked on the decaying of decl_ref_mut values to decl_refs when the decl goes out of scope. I'll try and implement that soon, excited to inadvertently break everyone's bad comptime code :)

For the time being, I'll mark this as a draft so maintainers know they don't need to look at it.

[andrewrk]

Closing abandoned draft