Port "Improve unknown narrowing by negated type predicates"

[Andarist]

ports microsoft/TypeScript#60795

[Copilot]

Pull Request Overview

Ports the enhanced narrowing behavior for unknown when using negated type predicates from upstream TypeScript.

• Removes the outdated .types.diff baseline file.
• Updates the expected outputs in narrowUnknownByTypePredicate.types for nullish and non-nullish predicates.
• Introduces handling of unknownUnionType and wraps filterType results in recombineUnknownType in flow.go.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
testdata/baselines/reference/submodule/compiler/narrowUnknownByTypePredicate.types.diff	Removed obsolete baseline diff file
testdata/baselines/reference/submodule/compiler/narrowUnknownByTypePredicate.types	Updated expected narrowing outputs for value2 and value4
internal/checker/flow.go	Added special unknown handling and recombination logic in flow.go

Comments suppressed due to low confidence (2)

testdata/baselines/reference/submodule/compiler/narrowUnknownByTypePredicate.types.diff:1

• The baseline diff file was removed; ensure that any test harness configuration or references to this file are also cleaned up to prevent stale entries.

diff --git a/testdata/baselines/reference/submodule/compiler/narrowUnknownByTypePredicate.types.diff b/testdata/baselines/reference/submodule/compiler/narrowUnknownByTypePredicate.types.diff

testdata/baselines/reference/submodule/compiler/narrowUnknownByTypePredicate.types:34

• Consider adding explicit unit tests or assertions for this updated narrowing behavior to safeguard against regressions in nullish and non-nullish cases.

>value2 : null | undefined