[ADT] Add BitVector::find_prev.

This almost completes the matrix of all possible find
functions.

*EXISTING*
----------
find_first
find_first_unset
find_next
find_next_unset
find_last
find_last_unset

*NEW*
----
find_prev

*STILL MISSING*
---------------
find_prev_unset

Differential Revision: https://reviews.llvm.org/D32885

llvm-svn: 302254