[dotest] Set environment variables after potentialy clearing others.
[dotest] Set environment variables after potentialy clearing others.

Dotest contains code to clear DYLD_LIBRARY_PATH for the inferior to not
propagate sanitized builds. However, it's possible that we want to
inject a different library path with `--inferior-env`. To make that work
correctly, we need to do that *after* clearing DYLD_LIBRARY_PATH.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@367054 91177308-0d34-0410-b5e6-96231b3b80d8

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