Update darwin SDK version parsing to support OSX and simulator

This re-commits r226005 with a tweak. The origin attempt failed because
Darwin bot sets up SDKROOT and clang can deduce SDK version from them
after this patch. That broke many driver tests due to the change of
deployment target version. Now the tests should not complain after
r240574.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240619 91177308-0d34-0410-b5e6-96231b3b80d8