Create the new linker type "linker_private_weak_def_auto".

It's similar to "linker_private_weak", but it's known that the address of the
object is not taken. For instance, functions that had an inline definition, but
the compiler decided not to inline it. Note, unlike linker_private and
linker_private_weak, linker_private_weak_def_auto may have only default
visibility.  The symbols are removed by the linker from the final linked image
(executable or dynamic library).

llvm-svn: 111684