Update stability-stdlib-abi.asserts.additional.swift.expected

This is a false positive, since the '_read' accessor is synthesized
on demand and emitted with shared linkage. See <rdar://problem/53776566>.

Author: slavapestov

test/api-digester/Outputs/stability-stdlib-abi.asserts.additional.swift.expected

@@ -6,3 +6,4 @@ Struct _GlobalRuntimeFunctionCountersState is a new API without @available attri
 Struct _ObjectRuntimeFunctionCountersState is a new API without @available attribute
 Struct _RuntimeFunctionCounters is a new API without @available attribute
 Func _measureRuntimeFunctionCountersDiffs(objects:_:) is a new API without @available attribute
+Accessor Slice.subscript(_:).Read() has been removed