[analyzer] Use inline storage in the FunctionSummary DenseMap.

The summaries lasted for the lifetime of the map anyway; no reason to
include an extra allocation.

Also, use SmallBitVector instead of BitVector to track the visited basic
blocks -- most functions will have less than 64 basic blocks -- and
use bitfields for the other fields to reduce the size of the structure.

No functionality change.

llvm-svn: 178514