Declarators can now properly represent template-ids, e.g., for

Declarators can now properly represent template-ids, e.g., for

  template void f<int>(int);
                ~~~~~~
Previously, we silently dropped the template arguments. With this
change, we now use the template arguments (when available) as the
explicitly-specified template arguments used to aid template argument
deduction for explicit template instantiations.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@82806 91177308-0d34-0410-b5e6-96231b3b80d8