Fix assertion failure left over from changes to move away from "darwin" triples.

I happened to notice this while trying to write a test for an iOS simulator
target. I suspect we just missed this when we added separate "macosx" and "ios"
triples instead of the generic "darwin" OS.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196527 91177308-0d34-0410-b5e6-96231b3b80d8