[llvm] [llvm-strings] Use small buffer instead of reading whole file (PR #163073)

[James Henderson via llvm-commits]

https://github.com/jh7370 commented:

> Hi. Sorry for the late reply. I was in vacation. Here is the PR uses cin. #215568

Thanks for this. I took a look and I don't think using `std::cin` really improves things, so we should follow the "no iostream" rule and stick with the version in this PR.

https://github.com/llvm/llvm-project/pull/163073