[Objective-C]. This patch extends  objc_bridge attribute to support objc_bridge(id).
This means that a pointer to the struct type to which the attribute appertains 
is a CF type (and therefore an Objective-C object of some type), but not of any 
specific class. rdar://19157264


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@224072 91177308-0d34-0410-b5e6-96231b3b80d8