Explicitly pass an isysroot to avoid the SDKROOT overriding the deployment target.
This fixes the green dragon builders after r284416.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@284423 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit fe83c63cfa5e1bba8ac4183d8be31d37fdb3aa2c)
1 file changed