Add aliases to ASTCodegen for dmd#8591

[thewilsonator]

So that #8591's dtoh can be built against an unmodified dmd (e.g as a dub package).

cc @ibuclaw

[dlang-bot]

Thanks for your pull request and interest in making D better, @thewilsonator! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Testing this PR locally

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dub fetch digger
dub run digger -- build "master + dmd#8776"