Rollup merge of #142377 - Urgau:unremap-rustc-dev, r=jieyouxu
Try unremapping compiler sources
See [#t-compiler/help > Span pointing to wrong file location (`rustc-dev` component)](https://rust-lang.zulipchat.com/#narrow/channel/182449-t-compiler.2Fhelp/topic/Span.20pointing.20to.20wrong.20file.20location.20.28.60rustc-dev.60.20component.29/with/521087083).
This PR is a follow-up to rust-lang/rust#141751 regarding the compiler side.
Specifically we now take into account the `CFG_VIRTUAL_RUSTC_DEV_SOURCE_BASE_DIR` env from rust-lang/rust#141751 when trying to unremap sources from `$sysroot/lib/rustlib/rustc-src/rust` (the `rustc-dev` component install directory).
Best reviewed commit by commit.
cc ``@samueltardieu``
r? ``@jieyouxu``
2d5b852f0a