[llvm-branch-commits] [libcxx] release/23.x: [libc++][format] Don't leave the output buffer full after a bulk write (#226791) (PR #226868)

[A. Jiang via llvm-branch-commits]

https://github.com/frederick-vs-ja approved this pull request.

I think we should backport this to the `release/23.x` branch, since this bug affects has been already impacting users for a long while and is possibly a regression as noted in https://github.com/llvm/llvm-project/pull/226791#pullrequestreview-5332961353 (and the original PR description before edits).

https://github.com/llvm/llvm-project/pull/226868