IR node @Reference is treated as @Ignore by the ASTWriter

Right now we print nothing if the ASTWriter encounters a node tagged as reference.

@Reference tagged nodes shouldn't be skipped, they should be toString:ed in the ASTWriter but not recursively traversed, as this is what leads to the infinite loop.

@Ignore should still be ignored.

We should also remove the @Ignore annotation for @References that we do want printed.