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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@95495 91177308-0d34-0410-b5e6-96231b3b80d8