Add code for emitting the attribute groups.

This is some initial code for emitting the attribute groups into the bitcode.

NOTE: This format *may* change! Do not rely upon the attribute groups' bitcode
not changing.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174845 91177308-0d34-0410-b5e6-96231b3b80d8