Rely on fewer features of the 'env' command. Darwin only supports '-i'.
I'm explicitly setting LC_ALL=C somewhat for documentation, but
hopefully this also removes some host variation from the test results.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223102 91177308-0d34-0410-b5e6-96231b3b80d8