Streamline `CrateMetadataRef` construction in `provide_one!`. `cstore.get_crate_data()` creates a `CrateMetadataRef`, which is exactly what we need. The current code is very confused and does several unnecessary things: mapping the `FreezeReadGuard` and calling `CStore::from_tcx` a second time to construct a second `CrateMetadataRef`. This is a small perf win. r? @mu001999