Fix declaring class template methods with an attributed typedef

This change unifies the logic for template instantiation of methods and
functions declared with typedefs.

It ensures that SubstFunctionType() always fills the Params out param
with non-null ParmVarDecls or returns null.

Reviewers: rsmith

Differential Revision: http://llvm-reviews.chandlerc.com/D1135

llvm-svn: 187528