compiler-builtins: pass -fdebug-compilation-dir=. to fix S_OBJNAME reproducibility on Windows

[paradoxicalguy]

compiler_builtins.rlib differs between builds because clang-cl embeds the absolute object output path into the S_OBJNAME codeview record.
the path comes from cc-rs passing an absolute /Fo argument (derived from cargo's OUT_DIR) to addDebugObjectName(), which llvm then writes verbatim into .debug$S.
minimal repro:

clang-cl /c /Z7 t.c /Fo:a\t.obj  →  ObjectName: C:\...\a\t.obj
clang-cl /c /Z7 t.c /Fo:b\t.obj  →  ObjectName: C:\...\b\t.obj

-fdebug-compilation-dir=. makes S_OBJNAME emit a relative path instead.
needs CI verification on a windows dist job since local builds use cl.exe rather than the bundled clang-cl. this pr is only for testing

[rustbot]

compiler-builtins is developed in its own repository. If possible, consider making this change to rust-lang/compiler-builtins instead.

cc @tgross35

[rustbot]

r? @jieyouxu

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[rustbot]

⚠️ Warning ⚠️

• The following commits have merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

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[paradoxicalguy]

cc @bjorn3 @Urgau

[bjorn3]

Reminder to remove this before merging.

View changes since the review

[paradoxicalguy]

yes, can we try a windows run on this?