resolve: Make disambiguators for underscore bindings module-local
Disambiguators attached to underscore name bindings (like `const _: u8 = something;`) do not need to be globally unique, they just need to be unique inside the module in which they live, because the bindings in a module are basically kept as `Map<BindingKey, SomeData>`.
Also, the specific values of the disambiguators are not important, so a glob import of `const _` may have a different disambiguator than the original `const _` itself.
So in this PR the disambiguator is just set to the current number of bindings in the module.
This removes one more piece of global mutable state from resolver and unblocks https://github.com/rust-lang/rust/pull/143884.
d3bc06ecb4