[clang] [libcxx] libcxx: std::ostream::sentry should be exported (PR #140169)

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jeremyd2019 wrote:

I know, I just started with #140145 for my testing, this pr is really meant for testing that no other platform is broken.  https://github.com/jeremyd2019/llvm-mingw/actions/runs/15075388808 is running the full test of [af82a52](https://github.com/llvm/llvm-project/pull/140169/commits/af82a52986d549e32a1384630d4a26b8ecfa7687)

https://github.com/llvm/llvm-project/pull/140169