[CodeGen] Mark weak vtables unnamed_addr on duplicable-vtable targets (#205930)

When a target's `getVTableUniqueness()` is `UniqueIfStrongLinkage`, as
on Apple Mach-O, the platform may emit a class's weak vtable as a
separate copy in more than one linkage unit. A symbol that can
legitimately exist at several addresses has no meaningful address, so
such a vtable can be marked `unnamed_addr`.

On Mach-O, a weak vtable that is `unnamed_addr` lowers to a
`.weak_def_can_be_hidden` definition, which the static linker coalesces
and then drops from the dynamic export trie. Keeping these vtables out
of the exported symbol set cuts the work dyld has to do at load time.

#200108 dropped `unnamed_addr` from vtables because the exact
`dynamic_cast` optimization compares an object's vtable pointer against
a known vtable address and depends on that address being unique. Adding
`unnamed_addr` is safe here because that optimization is never performed
for these weak vtables, so nothing relies on them having a unique
address.

rdar://179929460