This bug was found via self-build on lld, and worked around
The issue is that the 'using' causes the lookup to pick up the
first decl. However, when setting inherited default parameters,
we only update 'forward', not 'backward'. SO, only the newest param
list has all the information about the default arguments.
This patch ensures that the list of parameters we look through checks
the newest decl's template parameter list so it doesn't miss a default.