Fix --pretty=expanded with --remap-path-prefix

[jsgf]

Per #80832, using
--pretty=expanded and --remap-path-prefix results in an ICE.

This is becasue the session source files table is stored in remapped
form, whereas --pretty-expanded looks up unremapped files. This remaps
the path prefixes before lookup.

There don't appear to be any existing tests for --pretty=expanded; I'll look into
adding some. Never mind, found the pretty tests.

Fixes #80832

[rust-highfive]

r? @davidtwco

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[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

configure: rust.channel         := nightly
configure: rust.debug-assertions := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
configure: 
---
skip untracked path cpu-usage.csv during rustfmt invocations
skip untracked path src/doc/book/ during rustfmt invocations
skip untracked path src/doc/rust-by-example/ during rustfmt invocations
skip untracked path src/llvm-project/ during rustfmt invocations
Diff in /checkout/compiler/rustc_span/src/source_map.rs at line 1049:
                 let (path, mapped) = self.map_prefix(path.to_path_buf());
                 (FileName::Real(RealFileName::Named(path)), mapped)
             }
-            other => (other.clone(), false)
Running `"/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/rustfmt" "--config-path" "/checkout" "--edition" "2018" "--unstable-features" "--skip-children" "--check" "/checkout/compiler/rustc_span/src/source_map.rs"` failed.
If you're running `tidy`, try again with `--bless`. Or, if you just want to format code, run `./x.py fmt` instead.
+            other => (other.clone(), false),
     }
 }
failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test --stage 2 src/tools/tidy
Build completed unsuccessfully in 0:00:21

[davidtwco]

LGTM, r=me after minor change

[davidtwco]

Thanks!

@bors r+

[bors]

📌 Commit d7ce9d5 has been approved by davidtwco