Rollup merge of #35618 - jseyfried:ast_view_path_refactor, r=eddyb

Refactor `PathListItem`s

This refactors away variant `Mod` of `ast::PathListItemKind` and refactors the remaining variant `Ident` to a struct `ast::PathListItem_`.

diff --cc src/librustc/hir/fold.rs
Simple merge

diff --cc src/librustc/hir/lowering.rs
Simple merge

diff --cc src/librustc/hir/mod.rs
Simple merge

diff --cc src/librustc/hir/print.rs
Simple merge

diff --cc src/librustc/middle/dead.rs
Simple merge

diff --cc src/librustdoc/clean/mod.rs
Simple merge

diff --cc src/libsyntax/ast.rs
Simple merge

diff --cc src/libsyntax/fold.rs
Simple merge

diff --cc src/libsyntax/parse/parser.rs
Simple merge

diff --cc src/libsyntax/print/pprust.rs
Simple merge