Sema: Subst type default template args earlier

Summary:
We would not perform substitution at an appropriate point, allowing strange
results to appear. We would accepts things that we shouldn't or mangle things incorrectly.  Note that this hasn't fixed the other cases like
template-template parameters or non-type template parameters.

Reviewers: doug.gregor, rjmccall, rsmith

Reviewed By: rsmith

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1507