PR13386: When matching up parameters between a function template declaration

and a function template instantiation, if there's a parameter pack in the
declaration and one at the same place in the instantiation, don't assume that
the pack wasn't expanded -- it may have expanded to nothing. Instead, go ahead
and check whether the parameter pack was expandable. We can do this as a
side-effect of the work we'd need to do anyway, to find how many parameters
were produced.

llvm-svn: 160416