Add an ns_bridged attribute, used to specify that a

Add an ns_bridged attribute, used to specify that a
pointer to the annotated struct type can be used as an
Objective-C object pointer.  If an argument is given, the
type is actually "toll-free bridged" to the specific type
named there, rather than just to 'id'.

For now, we cannot rely on all types being so annotated,
and we'll always have to have exceptions for things like
CFTypeRef (aka const void*), but this is clearly a good
foundation for improving toolage in this area.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@140779 91177308-0d34-0410-b5e6-96231b3b80d8