Use a substituted type when determining how to substitute in non-type template

params.  Don't insert addrof operations when matching against a pointer;
array/function conversions should take care of this for us, assuming the
argument type-checked in the first place.  Add a fixme where we seem to be
using a less-restrictive reference type than we should.

Fixes PR 6249.

llvm-svn: 95495