[BitVector] Add find_[first,last]_[set,unset]_in.

A lot of code is duplicated between the first_last and the
next / prev methods.  All of this code can be shared if they
are implemented in terms of find_first_in(Begin, End) etc,
in which case find_first = find_first_in(0, Size) and find_next
is find_first_in(Prev+1, Size), with similar reductions for
the other methods.

Differential Revision: https://reviews.llvm.org/D33104

llvm-svn: 303269