Re-add mechanism to override LIT options using enviroment variables.
Re-add mechanism to override LIT options using enviroment variables.

There was a dumb mistake in the original commit that has now
been fixed

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@292351 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx