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``

Author: Urgau

src/tests/ui.md

@@ -113,6 +113,8 @@ Compiletest makes the following replacements on the compiler output:
 - The base directory where the test's output goes is replaced with
   `$TEST_BUILD_DIR`. This only comes up in a few rare circumstances. Example:
   `/path/to/rust/build/x86_64-unknown-linux-gnu/test/ui`
+- The real directory to the standard library source is replaced with `$SRC_DIR_REAL`.
+- The real directory to the compiler source is replaced with `$COMPILER_DIR_REAL`.
 - Tabs are replaced with `\t`.
 - Backslashes (`\`) are converted to forward slashes (`/`) within paths (using a
   heuristic). This helps normalize differences with Windows-style paths.