DebugInfo: Don't include implicit special members in the list of class members

By leaving these members out of the member list, we avoid them being
emitted into type unit definitions - while still allowing the
definition/declaration to be injected into the compile unit as expected.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219101 91177308-0d34-0410-b5e6-96231b3b80d8