[Support] Add support for finding unset bits in a BitVector.

BitVector had methods for searching for the first and next
set bits, but it did not have analagous methods for finding
the first and next unset bits.  This is useful when your ones
and zeros are grouped together and you want to iterate over
ranges of ones and zeros.

Differential Revision: https://reviews.llvm.org/D31802

llvm-svn: 299857