[llvm] [Support] mmap when possible in getSTDIN. (PR #162013)

[James Henderson via llvm-commits]

jh7370 wrote:

Make sure to do the two things in separate PRs, so that they can be reviewed on their own merits (you can keep this one for the non-llvm-strings case, if you want). You should also give performance examples of how the fixes improve memory consumption in some example cases. In the llvm-strings case, you probably also want to show the runtime impact of the overall change you make.

https://github.com/llvm/llvm-project/pull/162013