ObjectiveC. Implement attribute 'objc_bridge_mutable'
whose semantic is currently identical to objc_bridge,
but their differences may manifest down the road with
further enhancements. // rdar://15498044


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195376 91177308-0d34-0410-b5e6-96231b3b80d8