[Bitcode] Refactor attribute group writing to avoid getSlotAttributes

[Bitcode] Refactor attribute group writing to avoid getSlotAttributes

Summary:
That API creates a temporary AttributeList to carry an index and a
single AttributeSet. We need to carry the index in addition to the set,
because that is how attribute groups are currently encoded.

NFC

Reviewers: pcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32262

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301245 91177308-0d34-0410-b5e6-96231b3b80d8