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

[James Henderson via llvm-commits]

https://github.com/jh7370 commented:

According to the pre-merge checks, one of the tests is failing. Also, it would be worth adding some general comments to explain the different phases of the algorithm you're following (and why). This will help reduce the likelihood of performance regressions.

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