Fix panic in declaration emit for export default arrow/function expression with unnameable inferred return type - #63989

export default () => makeFoo() (or a function expression) whose inferred return type contains a symbol that cannot be named from the exporting module crashed the compiler with panic("Diagnostic emitted without context") instead of emitting TS4082, as TS 6.x did.

Root cause

• transformExportAssignment's ast.IsFunctionLike(unwrapped) branch promotes arrow/function-expression default exports to a function declaration via transformFunctionLikeToDeclaration → ensureType → serializeReturnTypeForSignature.
• Unlike the sibling non-function fallback branch a few lines below, this branch never installed a getSymbolAccessibilityDiagnostic context or pushed an error fallback node before serializing.
• When the checker reports a CannotBeNamed accessibility result during return-type serialization, it fell through to the file-level default handler, which is just panic(...).

Fix

• Install the same Default_export_of_the_module_has_or_is_using_private_name_0 diagnostic context and PushErrorFallbackNode(assignment)/PopErrorFallbackNode() pair around the function-like promotion, matching the parity already present in the fallback branch below it.

Tests

• Added declarationEmitDefaultExportArrowFunctionPrivateName.ts and declarationEmitDefaultExportFunctionExpressionPrivateName.ts, covering both export default () => ... and export default (function () {...}) forms with a cross-module unique-symbol-keyed member causing an unnameable inferred return type. Both now emit TS4082 instead of crashing.

• Fixes Panic "Diagnostic emitted without context" ts-go in declaration emit for export default arrow/function expression with non-portable inferred return type #63761