Rollup merge of #36438 - jseyfried:node_ids_in_expansion, r=nrc

Assign node ids during macro expansion

After this PR,
 - The `ExtCtxt` can access `resolve`'s `Resolver` through the trait object `ext::base::Resolver`.
  - The `Resolver` trait object can load macros and replaces today's `MacroLoader` trait object.
  - The macro expander uses the `Resolver` trait object to resolve macro invocations.
 - The macro expander assigns node ids and builds the `Resolver`'s `macros_at_scope` map.
   - This is groundwork for merging import resolution and expansion.
 - Performance of expansion together with node id assignment improves by ~5%.

**EDIT:** Since Github is reordering the commits, here is `git log`:
 - b54e1e399741579612f13e2df98a25ea9447989d: Differentiate between monotonic and non-monotonic expansion and only assign node ids during monotonic expansion.
 - 78c00398780db6f59ebf43e765fa9368dad436d2: Expand generated test harnesses and macro registries.
 - f3c2dca3539e6edc745f9c91898cb97d281865c1: Remove scope placeholders from the crate root.
 - c86c8d41a26b2037e80c9fd028a59313a78b3a66: Perform node id assignment and `macros_at_scope` construction during the `InvocationCollector` and `PlaceholderExpander` folds.
 - 72a636975fc5d0bb4af45af7bdd97987cc722a6a: Move macro resolution into `librustc_resolve`.
 - 20b43b23230ce063ccf99a4841d85790ad311bdf: Rewrite the unit tests in `ext/expand.rs` as a `compile-fail` test.
 - a9821e1658240bb2c056f260a4b6bc9789301fae: Refactor `ExtCtxt` to use a `Resolver` instead of a `MacroLoader`.
 - 60440b226d2f70bdae803443ff7ad2e2af2c9b10: Refactor `noop_fold_stmt_kind` out of `noop_fold_stmt`.
 - 50f94f6c95c944f08c4af264f48260e42efefd47: Avoid needless reexpansions.

r? @nrc

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