Darwin: fix stdlib handling when CLANG_DEFAULT_STDLIB is set

I'd accidentally skipped the CMake check in a premature optimisation. I'd also
put the original test in completely the wrong place.

Thanks Jonas Hahnfeld!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260898 91177308-0d34-0410-b5e6-96231b3b80d8