[lld/mac] Fix issues around thin archives

- most importantly, fix a use-after-free when using thin archives,
  by putting the archive unique_ptr to the arena allocator. This
  ports D65565 to MachO

- correctly demangle symbol namess from archives in diagnostics

- add a test for thin archives -- it finds this UaF, but only when
  running it under asan (it also finds the demangling fix)

- make forceLoadArchive() use addFile() with a bool to have the archive
  loading code in fewer places. no behavior change; matches COFF port a
  bit better

Differential Revision: https://reviews.llvm.org/D92360