Remove unsafe "__as_link()" cast member function.

"__as_link()" can only be used safely on "__list_node" objects. This patch
moves the "__as_link()" member function from "__list_node_base" to "__list_node"
so it cannot be used incorrectly.

Unsafe downcasts now use a non-member function so we don't defer the type-punned
pointer.

llvm-svn: 256727