Rollup merge of #63559 - eddyb:v0-mangling-off-by-1, r=estebank

Rollup merge of #63559 - eddyb:v0-mangling-off-by-1, r=estebank

rustc_codegen_utils: account for 1-indexed anonymous lifetimes in v0 mangling.

I don't really understand why `anonymize_late_bound_regions` starts with `BrAnon(1)` instead of `BrAnon(0)`, but it does (maybe @nikomatsakis knows?): https://github.com/rust-lang/rust/blob/c43d03a19f326f4a323569328cc501e86eb6d22e/src/librustc/ty/fold.rs#L696-L712

Thankfully, the mangling format and demangler implementations are fine, and I just needed to offset the anonymized lifetime indices by `1` to get the correct mangling.

cc @alexcrichton @michaelwoerister