[lit] Fix my earlier bogus fix to not set DYLD_LIBRARY_PATH with Asan.
[lit] Fix my earlier bogus fix to not set DYLD_LIBRARY_PATH with Asan.

My follow-up commit to mess with DYLD_LIBRARY_PATH was bogus for two
reasons:

 - The condition was inverted.
 - We were checking the OS's environment, instead of the config's.

Two wrongs don't make a right, but the second mistake meant that the
sanitizer bot passed.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@370483 91177308-0d34-0410-b5e6-96231b3b80d8

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