Turn the enum attributes DenseSet in AttrBuilder into a set of bits.

Avoids malloc and is a lot denser. We lose iteration over target independent
attributes, but that's a strange interface anyways and didn't have any users
outside of AttrBuilder.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175370 91177308-0d34-0410-b5e6-96231b3b80d8