Don't store AttributeSet::FunctionIndex as an int.

GCC complains: Core.cpp:1449:27: warning: overflow in implicit constant conversion [-Woverflow]
I'm not sure if that's really a problem here, but using the enum type is better
style anyways.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179696 91177308-0d34-0410-b5e6-96231b3b80d8