Initial implementation of function parameter packs. This implementation allows:

  1) Declaration of function parameter packs
  2) Instantiation of function parameter packs within function types.
  3) Template argument deduction of function parameter packs when
  matching two function types.

We're missing all of the important template-instantiation logic for
function template definitions, along with template argument deduction
from the argument list of a function call, so don't even think of
trying to use these for real yet.

llvm-svn: 122926