Bitcode: Simplify how we enumerate summaries in the index. NFCI.

Bitcode: Simplify how we enumerate summaries in the index. NFCI.

Instead of defining a custom iterator class, just use a function with a
callback, which is much easier to understand and less error prone.

Differential Revision: https://reviews.llvm.org/D32470

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301942 91177308-0d34-0410-b5e6-96231b3b80d8