Auto merge of #37931 - eddyb:meta-version, r=jseyfried

Auto merge of #37931 - eddyb:meta-version, r=jseyfried

rustc_metadata: don't break the version check when CrateRoot changes.

In #36551 I made `rustc_version` a field of `CrateRoot`, but despite it being the first field, one could still break the version check by changing `CrateRoot` so older compilers couldn't fully decode it (e.g. #37463).

This PR fixes #37803 by moving the version string back at the beginning of metadata, right after the 32-bit big-endian absolute position of `CrateRoot`, and by incrementing `METADATA_VERSION`.