Auto merge of #107055 - kylematsuda:eb-fn-sig, r=lcnr

Auto merge of #107055 - kylematsuda:eb-fn-sig, r=lcnr

Switch to `EarlyBinder` for `fn_sig` query

Part of the work to finish #105779 (also see https://github.com/rust-lang/types-team/issues/78).

Several queries `X` have a `bound_X` variant that wraps the output in [`EarlyBinder`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/subst/struct.EarlyBinder.html). This adds `EarlyBinder` to the return type of the `fn_sig` query and removes `bound_fn_sig`.

r? `@lcnr`