[llvm] [TableGen] Use a more efficient memory buffer for output (PR #123353)

[Jay Foad via llvm-commits]

jayfoad wrote:

This is mostly just an RFC to see if the idea makes sense, and if anything like my `stringvec_ostream` already exists? And if not, where should it live and what should it be called?

https://github.com/llvm/llvm-project/pull/123353