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 ⚠️

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  • 9f5dbc0

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[paradoxicalguy]

cc @bjorn3 @Urgau

[bjorn3]

@bors try job=dist-x86_64-msvc

[rust-bors]

☀️ Try build successful (CI)
Build commit: 5383ff6 (5383ff6bb690cf094a2399cf3e129092120625b2, parent: 9d862dd2c17b2dc90e9f623bb9279a8c4090b311)