[Darwin] Fix deployment target detection

There was a thinko in the deployment target detection code that
made the -isysroot parsing have precedence over the environment
variable for tvOS. This patch makes this logic symetric for all
platforms (the env variable must have precedence).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@257543 91177308-0d34-0410-b5e6-96231b3b80d8