Fix typo in ASTStructuralEquivalence.cpp for UnaryTransform types.

Previously UnaryTransformType nodes were comparing the same node
for structural equivalence. This was due to a typo where T1 was
on both sides of the comparison. This patch corrects that typo.

Unfortunately I couldn't find a way to test this change. It seems
that currently UnaryTransform nodes are never actually checked
for equivalence, only their canonical types are.

None the less, this correction seemed appropriate.

llvm-svn: 329151