Futureproof AttrBuild if we ever have more than 64 attr enum values.

Currently we're at 34. Bitset should compile into virtually the same code as
uint64_t here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175437 91177308-0d34-0410-b5e6-96231b3b80d8