Don't push relocation sections onto InputSection<ELFT>::Discarded.RelocSections

Summary:
LLVM3.3 (and earlier) would fail to include a relocation section in
the group that the section it was relocating is in. Object files
affected by this issue have been encountered in the wild when using LLD.

This would result in a siutation like:

  Section {
    Index: 5
    Name: .text._Z3fooIiEvv (6)
    Type: SHT_PROGBITS (0x1)
    Flags [ (0x206)
      SHF_ALLOC (0x2)
      SHF_EXECINSTR (0x4)
      SHF_GROUP (0x200)
    ]
    Address: 0x0
    Offset: 0x48
    Size: 5
    Link: 0
    Info: 0
    AddressAlignment: 1
    EntrySize: 0
  }
  Section {
    Index: 6
    Name: .rela.text._Z3fooIiEvv (1)
    Type: SHT_RELA (0x4)
    Flags [ (0x0)
    ]
    Address: 0x0
    Offset: 0x3F0
    Size: 24
    Link: 8
    Info: 5
    AddressAlignment: 8
    EntrySize: 24
  }

In LLD, during symbol resolution, we discard the section containing the
weak symbol, but this amounts to replacing it with
InputSection<ELFT>::Discarded.
When we later saw the corresponding relocation section, we would then
end up pusing onto InputSection<ELFT>::Discarded.RelocSections, which is
bogus.

Reviewers: ruiu, rafael

Subscribers: llvm-commits, Bigcheese

Differential Revision: http://reviews.llvm.org/D16898

llvm-svn: 259831