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

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

FYI, note that even if you include clang changes here, those won't be used by the libcxx CI build; the CI uses a prebuilt build of llvm-mingw - see `.github/workflows/libcxx-build-and-test.yaml`.

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