Implement LWG#2101 'Some transformation types can produce impossible types'  Introduced a new (internal) type trait '__is_referenceable' with tests. Use that trait in add_lvalue_reference, add_rvalue_reference and add_pointer.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@258418 91177308-0d34-0410-b5e6-96231b3b80d8