Use an AttrBuilder to generate the correct AttributeSet.

We no longer accept an encoded integer as representing all of the
attributes. Convert this via the AttrBuilder class into an AttributeSet with the
correct representation (an AttributeSetImpl that holds a list of Attribute
objects).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173750 91177308-0d34-0410-b5e6-96231b3b80d8