[llvm-objdump] Print "..." instead of random data for virtual sections

When disassembling with -D, skip virtual sections by printing "..." for
each symbol.

This patch also implements `MachOObjectFile::isSectionVirtual`.

Test case comes from:

```
.zerofill __DATA,__common,_data64unsigned,472,3
```

Differential Revision: https://reviews.llvm.org/D45824

llvm-svn: 330342