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

[Harald van Dijk via llvm-commits]

hvdijk wrote:

> We could verify this with perf, for example by looking at L1D cache misses or a similar counter. I can investigate this if you think it is necessary.

No, that's okay, thanks. If you do the measurements that jh7370 asked for and you don't see performance get worse, for me that's good enough.

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