[llvm-dwarfdump] Warn user when it encounters no null terminated strings.

When llvm-dwarfdump encounters no null terminated strings, we should
warn user about it rather than ignore it and print nothing.

Before this patch, when llvm-dwarfdump dumps a .debug_str section whose
content is "abc", it prints:

```
.debug_str contents:
```

After this patch:

```
.debug_str contents:
warning: no null terminated string at offset 0x0
```

Reviewed By: jhenderson, MaskRay

Differential Revision: https://reviews.llvm.org/D86998