ObjectiveC ARC. Introduce a new attribute, 'objc_bridge'
that teaches the compiler about a subset of toll-free 
bridging semantics. This is wip. // rdar://15454846


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194633 91177308-0d34-0410-b5e6-96231b3b80d8