Auto merge of #141730 - osiewicz:collect-crate-deps-postorder-use-indexset, r=nnethercote

cstore: Use IndexSet as backing store for postorder dependencies

`<rustc_metadata::creader::CStore>::push_dependencies_in_postorder` showed up in new benchmarks from rust-lang/rustc-perf#2143, hence I gave it a shot to remove an obvious O(n) there.

r? nnethercote

Author: bors