SI-6979 Small optimization in lub

If a member of `lubBase` is final, it cannot be
refined in the types we're lubbing.
Disable MIMA in master.

Binary compatibility checks do not make sense in master because
there's no base point against which we should check.
Merge pull request #1926 from gkossakowski/disable-mima-in-master

Disable MIMA in master.
Merge pull request #1908 from retronym/ticket/6979

SI-6979 Small optimization in lub