[libcxx-commits] [libcxx] [libc++] Remove usage of internal string function in sstream (PR #75858)

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AdvenamTacet wrote:

My change deleted `__str_.clear();` because it's already done in move constructor, but if we don't want to rely on this behavior, I should bring it back.

https://github.com/llvm/llvm-project/pull/75858