PR38136: improve handling of template argument deduction of non-trailing

PR38136: improve handling of template argument deduction of non-trailing
function parameter packs.

This makes our handling of non-trailing function parameter packs
consistent between the case of deduction at the top level in a function
call and other cases where deduction encounters a non-trailing function
parameter pack.

Instead of treating a non-trailing pack and all later parameters as
being non-deduced, we treat a non-trailing pack as exactly matching
any explicitly-specified template arguments (or being an empty pack
if there are no such arguments). This corresponds to the "never deduced"
rule in [temp.deduct.call]p1, but generalized to all deduction contexts.

Non-trailing template argument packs still result in the entire
template argument list being treated as non-deduced, as specified in
[temp.deduct.type]p9.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@336962 91177308-0d34-0410-b5e6-96231b3b80d8