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

[eddyb]

I don't really understand why anonymize_late_bound_regions starts with BrAnon(1) instead of BrAnon(0), but it does (maybe @nikomatsakis knows?):

rust/src/librustc/ty/fold.rs

Lines 696 to 712 in c43d03a

	/// Rewrite any late-bound regions so that they are anonymous. Region numbers are
	/// assigned starting at 1 and increasing monotonically in the order traversed
	/// by the fold operation.
	///
	/// The chief purpose of this function is to canonicalize regions so that two
	/// `FnSig`s or `TraitRef`s which are equivalent up to region naming will become
	/// structurally identical. For example, `for<'a, 'b> fn(&'a isize, &'b isize)` and
	/// `for<'a, 'b> fn(&'b isize, &'a isize)` will become identical after anonymization.
	pub fn anonymize_late_bound_regions<T>(self, sig: &Binder<T>) -> Binder<T>
	where T : TypeFoldable<'tcx>,
	{
	let mut counter = 0;
	Binder::bind(self.replace_late_bound_regions(sig, |_| {
	counter += 1;
	self.mk_region(ty::ReLateBound(ty::INNERMOST, ty::BrAnon(counter)))
	}).0)
	}

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

[rust-highfive]

r? @estebank

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[estebank]

@bors r+

[bors]

📌 Commit c1758d5 has been approved by estebank

[Centril]

@bors p=1