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

[James Henderson via llvm-commits]

jh7370 wrote:

Could I get some final performance numbers of this approach versus the old approach. Ideally, I'd like the following for both before and after the changes:
1. Linux, read from stdin
2. Linux, read from file specified on the command-line
3. Windows, read from stdin
4. Windows, read from file specified on the command-line
In all cases, I'd like both the timings and peak memory consumption of the process.

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