AttributeSetImpl: Summarize existing function attributes in a bitset.

The majority of attribute queries checks for the existence of an enum
attribute in the FunctionIndex slot. We only have 48 of those and can
therefore summarize them in an uint64_t bitset which measurably improves
compile time.

Differential Revision: http://reviews.llvm.org/D16618

llvm-svn: 259252