[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@352785
91177308-0d34-0410-b5e6-
96231b3b80d8