RFR: Downgrade subtyping/cast failures from errors to warnings

[Maurizio Cimadamore]

This PR reverts all changes to subtyping and cast conversion, meaning that assigning `null` to a null-restricted type is no longer an error.
I've resurrected the lint `null` category, to issue a warning in such cases.

There's some stuff left to do, as I realized that the compiler swallows null markers when emitting diagnostics. This is due to a conundrum in the diagnostic formatter calling `stripMetadata`, and that resulting in _all_ metadata (not just type annos) to be lost.

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Commit messages:
 - Remove spurious changes
 - Revert changes to stripMetadata & friends
 - Initial push

Changes: https://git.openjdk.org/valhalla/pull/1872/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1872&range=00
  Stats: 208 lines in 11 files changed: 138 ins; 49 del; 21 mod
  Patch: https://git.openjdk.org/valhalla/pull/1872.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1872/head:pull/1872

PR: https://git.openjdk.org/valhalla/pull/1872