Auto merge of #34904 - petrochenkov:rustcall, r=nikomatsakis

Auto merge of #34904 - petrochenkov:rustcall, r=nikomatsakis

Properly feature gate all unstable ABIs

Fixes https://github.com/rust-lang/rust/issues/34900
[breaking-change]
r? @pnkfelix

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Function-visiting machinery for AST/HIR is surprisingly error-prone, it's *very* easy to miss some cases or visit something twice while writing a visitor. This is the true problem behind https://github.com/rust-lang/rust/issues/34900. I'll try to restructure these visitors a bit and send one more PR later.