[Index/DocComments] When cloning a full comment, pick the template parameters from the original FullComment,
if the parameters are not already set (e.g. because it is a typedef).

Fixes crash of rdar://16128173

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207382 91177308-0d34-0410-b5e6-96231b3b80d8