Fix Debug Failure crash when importing from mapped type module exports

[veeceey]

Fixes #58534

Problem

When importing a named export from a module that re-exports via a mapped type (e.g. Omit<typeof ns, never>), the compiler crashes with Error: Debug Failure inside getTypeOfVariableOrParameterOrPropertyWorker.

Minimal reproduction:

// assert.d.ts
declare namespace assert {
  class AssertionError extends Error {}
  const strict: Omit<typeof assert, never>;
}
export = assert.strict;

// index.ts
import { AssertionError } from "./assert";
new AssertionError("assert"); // Debug Failure crash

This also affects real-world code importing AssertionError from node:assert/strict (with @types/node).

Root Cause

combineValueAndTypeSymbols creates a combined symbol for names that resolve to both a value and a type. When the value symbol is a property of a mapped type (has CheckFlags.Mapped), the function was not preserving the mapped symbol's check flags or link properties (mappedType, keyType).

This caused getTypeOfSymbol to skip the CheckFlags.Mapped branch and fall through to getTypeOfVariableOrParameterOrProperty, which then hit Debug.assertIsDefined(symbol.valueDeclaration) — mapped symbols don't have a valueDeclaration, so this crashed.

Fix

Propagate CheckFlags.Mapped and the mapped symbol link properties (mappedType, keyType, nameType, syntheticOrigin) from the value symbol to the combined symbol in combineValueAndTypeSymbols. This ensures getTypeOfSymbol correctly dispatches to getTypeOfMappedSymbol for the combined symbol.

The fix is intentionally conservative — only CheckFlags.Mapped is propagated, not other check flags that might require different symbol link structures.

[veeceey]

checking in on this — let me know if there's anything I should update

[typescript-bot]

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