Rough out a new c'tor for the AttrBuilder class.

This c'tor takes the AttributeSet class as the parameter. It will eventually
grab the attributes from the specified index and create a new attribute builder
with those attributes.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171712 91177308-0d34-0410-b5e6-96231b3b80d8