Remove some introspection functions.

The 'getSlot' function and its ilk allow introspection into the AttributeSet
class. However, that class should be opaque. Allow access through accessor
methods instead.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173522 91177308-0d34-0410-b5e6-96231b3b80d8