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

[via llvm-commits]

aokblast wrote:

This is the CI result for removing trailing zero check.
https://github.com/llvm/llvm-project/actions/runs/18273318718/job/52019747511
It seems that Linux failed in a DAP test.
And windows failed in llc, however, the test results in Windows shows no symbol name and I don't have a Windows machine for debugging.

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