[llvm-nm] - Move MachO specific logic out from the dumpSymbolNamesFromObject(). NFC.

`dumpSymbolNamesFromObject` is the method that dumps symbol names.

It has 563 lines, mostly because of huge piece of MachO specific code.
In this patch I move it to separate helper method.

The new size of `dumpSymbolNamesFromObject` is 93 lines. With it it becomes
much easier to maintain it.

I had to change the type of 2 name fields to `std::string`, because MachO logic
uses temporarily buffer strings (e.g `ExportsNameBuffer`, `BindsNameBuffer` etc):

```
  std::string ExportsNameBuffer;
  raw_string_ostream EOS(ExportsNameBuffer);
```

these buffers were moved to `dumpSymbolsFromDLInfoMachO` by this patch and
invalidated after return. Technically, before this patch we had a situation
when local pointers (symbol names) were assigned to members of global static `SymbolList`,
what is dirty by itself.

Differential revision: https://reviews.llvm.org/D94667