Motivation for this change

See linked issue for discussion,
but key bits:

• rustc breaks without this
• fix changes ABI, may become 7.1.0

My "day-job" project broke with LLVM 7
until I found my way to this, d'oh!

I don't think we need to worry about
ABI the same way the folks on the issue do.

(and aligning with GCC ABI is much more important
than ABI-compat with previous LLVM major)

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

BTW debugging this is kinda neat in a painful way,
as the value --without any writes or instructions--
appears to corrupt itself when passed as an argument.
(it doesn't change but your debugger might say it does!)