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

[via llvm-commits]

aokblast wrote:

Currently, some programs rely *c = '\0' to find the end of a buffer.
In my experiment, therefore, it is impossible for us to mmap  in all cases since some file may not have '\0' in the end of the file and requires copy buffer and append '\0' manually.
However, it would be a huge performace if we use mmap when processing a huge file.
Can I rely on the test-suite result to make sure which program rely on *c = '\0' and therefore provide mmap acceleration for other programs?

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