Update installed compiler dependencies

[andjo403]

As llvm have updated the minimum toolchain
https://reviews.llvm.org/D66188

[rust-highfive]

r? @nikomatsakis

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[nikic]

The bump only happened for LLVM 10 (while we're on LLVM 9), so technically we still support older compiler versions at this point in time.

[andjo403]

but to get it to work you need to set the allow-old-toolchain in the config.toml for llvm to receive LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN shall we add that to the description then?

[JohnCSimon]

Ping from triage -
Hello, this PR has sat idle for eight days, what else needs to happen to resolve this?
@nikic @andjo403 @nikomatsakis

Thank you.

[andjo403]

maybe I shall point to the https://reviews.llvm.org/D57264 that is in the llvm version used and due to that commit there is an compile error if the LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN option is not set and as that is not described in the readme I think update the compiler version is a good solution when llvm 10 also removes that possibility anyway.

[nikomatsakis]

I think it makes sense to list the more modern dependencies as the default, and perhaps add a "subbullet" or something saying "you can use the older versions if you set LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN". (IIUC, that env variable is needed to use the older versions?)

[andjo403]

added a description in new chapter "Building rustc with older host toolchains"

[JohnCSimon]

Ping from triage -
Hi! @andjo403 , can you please address the change request from @nikomatsakis?

Thank you.

[andjo403]

update the text according to the comments but also removed the range of compilers that was listed before due to the higher version listed did not need the option to be set

[nikomatsakis]

@bors r+ rollup

Thanks @andjo403

[bors]

📌 Commit 166719a has been approved by nikomatsakis