Similarly to the compile unit lists, the list of types can also be
managed by the symbol file itself.
Since the only purpose of this list seems to be to maintain an owning
reference to all the types a symbol file has created (items are only
ever added to the list, never retrieved), I remove the passthrough
functions in SymbolVendor and Module. I also tighten the interface of
the function (return a reference instead of a pointer, make it protected
instead of public).