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

[via llvm-commits]

aokblast wrote:

One thing that I am not sure  is if we need to overwrite new operator like new (NamedBufferAlloc(BufferName)). If so, what is the proper BufferName? stdin?

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