Fix narrowing of generic this parameters

[Mateusz Burzyński (Andarist)]

fixes #64186

[Copilot]

🟢 Approval recommended

The focused checker change consistently matches parameter behavior and is covered by comprehensive regression baselines.

Pull request overview

Aligns control-flow narrowing of generic this parameters with ordinary parameters, fixing #64186.

Changes:

• Applies narrowable constraint types to this references while preserving check modes.
• Adds regression tests for direct, aliased, contextual, indexed, and inferential usage.
• Adds diagnostic, type, and symbol baselines.

File summaries

File	Description
tsc/internal/checker/checker.go	Enables consistent narrowing for generic this.
tsc/testdata/tests/cases/compiler/controlFlowGenericThis.ts	Tests core narrowing behavior.
tsc/testdata/tests/cases/compiler/controlFlowGenericThisContextual.ts	Tests contextual and inferential behavior.
tsc/testdata/baselines/reference/compiler/controlFlowGenericThis.errors.txt	Records expected diagnostics.
tsc/testdata/baselines/reference/compiler/controlFlowGenericThis.types	Records inferred types.
tsc/testdata/baselines/reference/compiler/controlFlowGenericThis.symbols	Records resolved symbols.
tsc/testdata/baselines/reference/compiler/controlFlowGenericThisContextual.types	Records contextual inferred types.
tsc/testdata/baselines/reference/compiler/controlFlowGenericThisContextual.symbols	Records contextual symbols.

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 0
• Review effort level: Balanced

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