In a partial instantiation of a pack expansion we can have a situation
where we have arguments for some of the packs but not all.  We deal
with this by leaving the pattern uninstantiated and remembering the
arguments passed to the partial specialization so we can use them
later in doing a full instantiation.  My patch for 84036 started to
use this mechanism when we don't have the expansion of a function
parameter pack, but it broke for this testcase because when we went to
look for the local specialization of the function parameter pack, we
were looking for a specialization of the original parameter pack, but
local_specializations only knows about the partially instantiated
parameter pack.  Fixed by remembering in this situation what function
we are using parameter packs from, so we can set up the appropriate
mapping.

Tested x86_64-pc-linux-gnu, applying to trunk.