AttributeSetNode: Summarize existing attributes in a bitset.

The majority of queries just checks for the existince of an enum
attribute.  We only have 48 of those and can summaryiz them in an
uint64_t bitfield so we can avoid searching the list. This improves
"opt" compile time by 1-4% in my measurements.

Differential Revision: http://reviews.llvm.org/D16617

llvm-svn: 259251