fix rdar://9780211 - Clang crashes with an assertion failure building WKView.mm from WebKit

This is something of a hack, the problem is as follows:

1. we instantiate both copied of RetainPtr with the two different argument types
   (an id and protocol-qualified id).
2. We refer to the ctor of one of the instantiations when introducing global "x",
   this causes us to emit an llvm::Function for a prototype whose "this" has type
   "RetainPtr<id<bork> >*".
3. We refer to the ctor of the other instantiation when introducing global "y",
   however, because it *mangles to the same name as the other ctor* we just use
   a bitcasted version of the llvm::Function we previously emitted.
4. We emit deferred declarations, causing us to emit the body of the ctor, however
   the body we emit is for RetainPtr<id>, which expects its 'this' to have an IR
   type of "RetainPtr<id>*".

Because of the mangling collision, we don't have this case, and explode.

This is really some sort of weird AST invariant violation or something, but hey
a bitcast makes the pain go away.

llvm-svn: 135572