[llvm] [Support] Treat Windows "nul" as the null device in writeToOutput (PR #208179)

[Wenju He via llvm-commits]

wenju-he wrote:

> If the device's canonical name is NUL (uppercase), Use NUL instead of nul in the code.

thanks for the advice. Changed to canonical name NUL in the code

https://github.com/llvm/llvm-project/pull/208179