Get rid of the 'Bits' mask in the attribute builder.

Get rid of the 'Bits' mask in the attribute builder.

The bit mask thing will be a thing of the past. It's not extensible enough. Get
rid of its use here. Opt instead for using a vector to hold the attributes.

Note: Some of this code will become obsolete once the rewrite is further along.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171553 91177308-0d34-0410-b5e6-96231b3b80d8