Properly convert all declaration non-type template arguments when

Properly convert all declaration non-type template arguments when
forming non-type template parameter values.

This reverts commit 93cc9dddd82f9e971f382ade6acf6634c5914966,
which reverted commit 11d10527852b4d3ed738aa90d8bec0f398160593.

We now always form `&x` when forming a pointer to a function rather than
trying to use function-to-pointer decay. This matches the behavior of
the old code in this case, but not the intent as described by the
comments.