Missing unsafe blocks in anonymous field helpers with compatible-codegen

[BigWingBeat]

If the compatible-codegen config option is enabled, the anonymous field helper ref properties (Those disabled by exclude-anonymous-field-helpers) are generated with a fixed block and pointer member accesses, but without a corresponding unsafe block, resulting in the generated code failing to compile with CS0214: Pointers and fixed size buffers may only be used in an unsafe context.