Properly skip EnumForwardDecls even with attributes

[christiangnrd]

MacOS ObjectiveC headers have enum forward declarations with attributes.

Currently, such a node gets marked as skip during CodegenPreprocessing:

Clang.jl/src/generator/passes.jl

Lines 632 to 638 in 572eddf

	if skip_check(dag, node)
	skip_mode = is_dup_tagtype(node) ? SoftSkip() : Skip()
	skip_mode == Skip() && push!(x.skip_nodes, i)
	dag.nodes[i] = ExprNode(node.id, skip_mode, node.cursor, node.exprs, node.adj)
	show_info &&
	@info "[CodegenPreprocessing]: skip a $(node.type) node named $(node.id)"
	end

but then it immediately gets replaced with an EnumLayout due to the next check checking the original node:

Clang.jl/src/generator/passes.jl

Lines 639 to 644 in 572eddf

	if attribute_check(dag, node)
	ty = attribute_type(node.type)
	dag.nodes[i] = ExprNode(node.id, ty, node.cursor, node.exprs, node.adj)
	show_info &&
	@info "[CodegenPreprocessing]: mark an attribute $(node.type) node named $(node.id)"
	end

I'm not sure if my fix is the intended behaviour, so it would be nice to have someone review it. Other solutions I consedered were adding a continue to the end of each check (or only to skip_check), or maybe instead of doing checks on the original node, always check the newest.

Includes changes from #519.