Resubmit "[BitVector] Add operator<<= and operator>>=."

Resubmit "[BitVector] Add operator<<= and operator>>=."

This was failing due to the use of assigning a Mask to an
unsigned, rather than to a BitWord.  But most systems do not
have sizeof(unsigned) == sizeof(unsigned long), so the mask
was getting truncated.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@300857 91177308-0d34-0410-b5e6-96231b3b80d8