This is easier for tooling to handle than trying to reverse-engineer it from the filename extension.

The commit moves metadata writing from `link_binary` to
`encode_metadata` (and renames the latter as
`encode_and_write_metadata`). This is at the very start of code
generation.

This commit separates metadata encoding (`tcx.encode_metadata`) from the
creation of the metadata module (which is now handled by
`write_compressed_metadata`, formerly `write_metadata`).

The metadata encoding now occurs slightly earlier in the pipeline, at
the very start of code generation within `start_codegen`.

Metadata *writing* still occurs near the end of compilation; that will
be moved forward in subsequent commits.

Related to #58372
Related to #58967