Sanitizers support

[tmiasko]

Documentation for existing sanitizers support in rustc, as well as changes proposed in rust-lang/rust#65241.

[mark-i-m]

Thanks!

One minor nit, if you have time

[mark-i-m]

Perhaps make the title a markdown heading or bolded or something?

[tmiasko]

Added an introductory paragraph to examples and additional headings.

[tmiasko]

I would probably wait for mentioned PR in rustc to get merged first. While
almost everything described here is already true, the recommendation about using
-Zbuild-std isn't.

[nikomatsakis]

This is awesome! That said, I wonder if this documentation would be better placed in the rustc book, which can be found (I think) in src/doc/rustc. What I would expect to include here in the rustc-guide are a few details on how it is implemented. Basically trying to convey the high-level strategy that is implemented in this commit.

[tmiasko]

I extended "How are sanitizers implemented in rustc?" section, and removed everything else.

[mark-i-m]

Thanks!

[mark-i-m]

It looks like CI is failing

[nikomatsakis]

Wow this is so great, thank you thank you. =)

[nikomatsakis]

For some reason I can't make suggestions on this repo, but this should be

please refer to [the unstable book](https://doc.rust-lang.org/unstable-book/).

[nikomatsakis]

Nit: executon of the following steps

[nikomatsakis]

Nit: the compiler-rt project

[tmiasko]

Thanks. I applied the suggestions.

[mark-i-m]

Thanks!