[HIP] Use nul instead of /dev/null when running on windows
[HIP] Use nul instead of /dev/null when running on windows

When clang is running on windows, /dev/null is not available. Use nul as empty input file instead.

Differential Revision: https://reviews.llvm.org/D56225


git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@350885 91177308-0d34-0410-b5e6-96231b3b80d8

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