Handling remaining rule for synthesize bitfields in

Handling remaining rule for synthesize bitfields in
class extensions (nonfragile-abi2).For every class @interface and class
extension @interface, if the last ivar is a bitfield of any type,
then add an implicit `char :0` ivar to the end of that interface.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@111857 91177308-0d34-0410-b5e6-96231b3b80d8