[ELF] Return the section name when calling getSymbolName on a section symbol.

Summary:
Previously, llvm-nm would report symbols for .debug and .note sections as: '?' with an empty  section name:

```
00000000 ?
00000000 ?
...
```

With this patch the output more closely resembles GNU nm:
```
00000000 N .debug_abbrev
00000000 n .note.GNU-stack
...
```

This patch calls `getSectionName` for sections that belong to symbols of type `ELF::STT_SECTION`, which returns the name of the section from the section string table.

Reviewers: Bigcheese, davide, jhenderson

Reviewed By: davide, jhenderson

Subscribers: rupprecht, jhenderson, llvm-commits

Differential Revision: https://reviews.llvm.org/D57105

llvm-svn: 352785