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

[James Henderson via llvm-commits]

https://github.com/jh7370 approved this pull request.

LGTM, with one remaining comment about the test file extension.

I ran this locally and could see significant peak working set improvements on Windows when simply looking at the working set size reported by Resource Monitor. For my arbitrary large test input, it went from needing well over 1GB of memory to 6KB, for both reading direct from file and stdin cases.

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