[lit] Fix the way we check if an environment var is set
[lit] Fix the way we check if an environment var is set

The old method would throw a KeyError.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@370138 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb