Remove all usages of "const" node pointer typedefs in the assoc containers.

The "const" pointer typedefs such as "__node_const_pointer" and
"__node_base_const_pointer" are identical to their non-const pointer types.
This patch changes all usages of "const" pointer type names to their respective
non-const typedef.

Since "fancy pointers to const" cannot be converted back to a non-const pointer
type according to the allocator requirements it is important that we never
actually use "const" pointers.

Furthermore since "__node_const_pointer" and "__node_pointer" already
name the same type, it's very confusing to use both names. Especially
when defining const/non-const overloads for member functions.

llvm-svn: 261419