Implement class-pre-define time redefinition.

This allows one to redefine classes as they are being loaded without
restriction as to the types of transformations that may be included.
For example one is allowed to add additional methods or fields to the
class being defined.

Bug: 31684920
Test: mma -j40 test-art-host
Change-Id: I671ee13444c05f28424e727fd80d9d46c78ca287