Support a rustup-toolchain-install-master contribution workflow for rustdoc

[Manishearth]

Currently to hack on rustdoc you need to build all of rustc, which takes forever and hinders contribution.

It might be better to do what clippy does: use rustup-toolchain-install-master

Currently this works on rustdoc with the following steps:

# setup
cargo install rustup-toolchain-install-master
rustup-toolchain-install-master -n master -c rustc-dev -f

# remove workspace file so that we don't mess with rustbuild
rm Cargo.toml

cd src/tools/rustdoc
cargo +master build
./target/debug/rustdoc

cc @GuillaumeGomez @QuietMisdreavus @Mark-Simulacrum

It may also be worth making this kind of workflow a more first-class part of x.py.

[Mark-Simulacrum]

This was discussed at the all hands this year wrt to integration into x.py. I've not yet had the time/energy to sit down and do it, even though it is not very difficult I expect. I should probably try and write up mentoring instructions.

[Manishearth]

In the meantime it may be worth investigating the RTIM solution (importantly, how to use this compiled rustdoc, rustup run master ./target/debug/rustdoc foo.rs) and documenting it

[jyn514]

I'm working on this in #79540.

[jyn514]

This has been subsumed by download-rustc I think; I don't have any plans to make this work with rust-toolchain directly.