Rewrite the removeAttr() method.

This now uses the AttributeSet object instead of the Attribute /
AttributeWithIndex objects. It's fairly simple now. It goes through all of the
subsets before the one we're modifying, adds them to the new set. It then adds
the modified subset (with the requested attributes removed). And then adds the
rest of the subsets.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173660 91177308-0d34-0410-b5e6-96231b3b80d8