Add CreateAliasVariation

CreateAliasVariation creates a new variation for the current mutator
that is an alias to a variation that was created with CreateVarations
by the same mutator.  It is similar to AliasVariation, except that
AliasVariation creates an alias from the pre-mutated variant to one
of the new variations, while CreateAliasVariation creates an alias
from a new post-mutated variation to one of the new variations.

Bug: 164216768
Test: TestCreateAliasVariation
Change-Id: I8847b8d6fc1d3248abc910b2fe2399881f9bdaee