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.