Auto merge of #71858 - petrochenkov:env, r=Mark-Simulacrum

Print environment variables accessed by rustc as special comments into depinfo files

So cargo (and perhaps others tools) can use them for linting (at least) or for actually rebuilding crates on env var changes.

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I've recently observed one more forgotten environment variable in a build script https://github.com/rust-lang/rust/pull/71314/commits/8a77d1ca3fc2df789157f7986ddbaf2a377ff0fe and thought it would be nice to provide the list of accessed variables to cargo automatically as a part of depinfo.

Unsurprisingly, I wasn't the first who had this idea - cc https://github.com/rust-lang/rust/issues/70517 https://github.com/rust-lang/rust/issues/40364 https://github.com/rust-lang/rust/issues/44074.

Also, there are dozens of uses of `(option_)env!` in rustc repo and, like, half of them are not registered in build scripts.

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Description:
- depinfo files are extended with special comments containing info about environment variables accessed during compilation.
- Comment format for environment variables with successfully retrieved value: `# env-dep:KEY=VALUE`.
- Comment format for environment variables without successfully retrieved value: `# env-dep:KEY` (can happen with `option_env!`).
- `KEY` and `VALUE` are minimally escaped (`\n`, `\r`, `\\`) so they don't break makefile comments and can be unescaped by anything that can unescape standard `escape_default` and friends.

FCP report: https://github.com/rust-lang/rust/pull/71858#issuecomment-633071488

Closes https://github.com/rust-lang/rust/issues/70517
Closes https://github.com/rust-lang/rust/issues/40364
Closes https://github.com/rust-lang/rust/issues/44074
A new issue in the cargo repo will be needed to track the cargo side of this feature.

r? @ehuss

diff --cc src/librustc_interface/passes.rs
Simple merge

diff --cc src/librustc_session/parse.rs
Simple merge