Provides a variant of the UnimplementedError macro specifically for use in incomplete target instruction lowering.  When --skip-unimplemented is specified, the UnimplementedLoweringError macro adds FakeUse and FakeDef instructions in order to maintain consistency in liveness analysis.

BUG= none
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/1591893002 .