Fix documentation warnings in ConstantPool

[abrown]

Prior to this commit I would see warnings like the following when running cargo +nightly doc:

warning: `[ir::ConstantData]` cannot be resolved, ignoring it...
  --> cranelift-codegen/src/ir/constant.rs:49:22
   |
49 | /// [`ConstantData`](ir::ConstantData)).
   |                      ^^^^^^^^^^^^^^^^ cannot be resolved, ignoring
   |
   = note: `#[warn(intra_doc_link_resolution_failure)]` on by default
   = help: to escape `[` and `]` characters, just add '\' before them like `\[` or `\]`

I believe the trick is to prepend crate (and not cranelift_codegen or a relative path) for references inside the file's own crate.

[bnjbvr]

Thanks!