Run our instrumentation after dfsan

[mcopik]

We decided to run dfsan after our instrumentation because we didn't know how to deal with dfsan's optimization that store labels in register variables instead of memory. Now we know right API calls and we could resolve this issue.

The current setting, while it works, leads to problems on LLVM builds with enabled assertions. There, dfsan performs some standard sanity checks and detects that we retval TLS variable to detect taint variable of called function.

• Add a test that leads to dfsan optimizing and storing taint value locally
• Change our taint reading function to use dfsan internal API
• Change the order of instrumentation

[mcopik]

It seems that dfsan pass needs to be run to resolve dfsan_get_label calls. Perhaps we can not use explicitly TLS variables of return value from a function?