Debug Info: Silently accept template argument packs

We could support the GCC extension DW_TAG_GNU_template_parameter_pack if
we're feeling adventurous, at some point - but I don't think GDB's doing
anything useful with it yet anyway.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181644 91177308-0d34-0410-b5e6-96231b3b80d8