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

[Harald van Dijk via llvm-commits]

hvdijk wrote:

> How did you test the file without mmap? Maybe I can try to reproduce this in my another patch.

Sorry for the confusion, I meant I compared LLVM main (processing the whole file in one go using `mmap`) to your branch (processing the file in chunks without `mmap`). I did not measure the performance of reading the whole file info memory without using `mmap`.

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