Once we've collected the template arguments for a
partially-substituted parameter pack in a template, forget about the
partially-substituted parameter pack: it is now completed. Fixes
<rdar://problem/12176336>.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172859 91177308-0d34-0410-b5e6-96231b3b80d8