Improve template instantiation for object constructions in several ways:

  - During instantiation, drop default arguments from constructor and
    call expressions; they'll be recomputed anyway, and we don't want
    to instantiate them twice.
  - Rewrote the instantiation of variable initializers to cope with
    non-dependent forms properly.

Together, these fix a handful of problems I introduced with the switch
to always rebuild expressions from the source code "as written."

llvm-svn: 91315