Add support in the bitcode reader to read the attribute groups.

This reads the attribute groups. It currently doesn't do anything with them.

NOTE: In the commit to the bitcode writer, the format *may* change in the near
future. Which means that this code would also change.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174849 91177308-0d34-0410-b5e6-96231b3b80d8