Avoid double lowering of generic identifiers.

`lower_generic_bound_predicate` calls `lower_ident`, and then passes the
lowered ident into `new_named_lifetime`, which lowers it again. This
commit avoids the first lowering. This requires adding a `lower_ident`
call on a path that doesn't involve `new_named_lifetime`.

Author: nnethercote

compiler/rustc_ast_lowering/src/item.rs

@@ -1720,7 +1720,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         let bounds = self.lower_param_bounds(bounds, itctx);
 
-        let ident = self.lower_ident(ident);
         let param_span = ident.span;
 
         // Reconstruct the span of the entire predicate from the individual generic bounds.
@@ -1739,6 +1738,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let def_id = self.local_def_id(id).to_def_id();
                 let hir_id = self.next_id();
                 let res = Res::Def(DefKind::TyParam, def_id);
+                let ident = self.lower_ident(ident);
                 let ty_path = self.arena.alloc(hir::Path {
                     span: param_span,
                     res,