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

[Harald van Dijk via llvm-commits]

hvdijk wrote:

> For some reason, we don't use mmap even in normal cases.

We do when `llvm-strings` opens a file specified by name in the command line arguments, rather than reading from stdin. But if your measurements show that it doesn't have a big impact on performance anyway, that's great, in that case it doesn't matter if we stop doing that.

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