Add comment to explain a non-obvious setting; NFC.

This is paraphrased from Simon Pilgrim's comment in:
http://reviews.llvm.org/D7492



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229566 91177308-0d34-0410-b5e6-96231b3b80d8
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