[llvm] [cmake] Don't generate per-file "__SHORT_FILE__" defines on visual studio builds (PR #151167)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Yes, it demonstrates my lack of understanding of what actually goes on in cmake vs builds - all I can see is that there is at most 2 cores being used for cl.exe and cmake.exe keeps reappearing as well.

https://github.com/llvm/llvm-project/pull/151167