Downgrade subtyping/cast failures from errors to warnings

[mcimadamore]

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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Progress

• Change must not contain extraneous whitespace

Reviewers

• Vicente Romero (@vicente-romero-oracle - Committer)

Reviewing

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[mcimadamore]

Note the ! here -- it's an hack, since the compiler swallows it :-(

[vicente-romero-oracle]

hehe, good one

[bridgekeeper]

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[openjdk]

@mcimadamore This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

Downgrade subtyping/cast failures from errors to warnings

Reviewed-by: vromero

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[mlbridge]

Webrevs

• 00: Full (ea2a72b5)

[vicente-romero-oracle]

lgtm

[mcimadamore]

/integrate

[openjdk]

Going to push as commit 0920450.

[openjdk]

@mcimadamore Pushed as commit 0920450.

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