[llvm] [lit] Use provided streams in builtin diff (PR #204869)

[Nathan Corbyn via llvm-commits]

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cofibrant wrote:

This change introduces some build failures on our bots: https://ci.swift.org/job/llvm.org/job/clang-stage1-RA-as/job/main/2540/

https://github.com/llvm/llvm-project/pull/204869