...when t1 is ElaboratedType sugar (which isn't only used when the type is
written with an elaborated type keyword, but also when it is written with a
qualified name).

(I originally wrote testArithmeticTypedefs to track down a different issue,
which turned out to be a non-issue, with this fix as fall-out.  So that test
doesn't quite match the theme of this commit, but is a worthwhile addition
nonetheless.)

Change-Id: Ic447da4399853d7d045e3e2e7ade8ddf52d89749