ADT: Use std::list in SparseBitVector, NFC

The only intrusive thing about SparseBitVector's usage of ilist<> was
that new was usually called externally.  There were no custom traits.

It seems like the reason to switch to ilist in r41855 was to avoid
pointer invalidation, but std::list<> has that feature too.  Maybe
std::list<>::emplace makes this a little more obvious than it was then.

Switch over to std::list<> and simplify the code.

llvm-svn: 280573