LSan can be combined with a parent tool (for now it's only ASan).
Also, we allow LSAN_OPTIONS to override certain common flags. It means
we have to parse LSAN_OPTIONS early enough, before the rest of the
parent tool (including chunks of sanitizer_common) is initialized.
In future, we can use the same approach for UBSan, after we embed it
into ASan runtime in a similar way.