objective-c: Normally, a property cannot be both 'readonly' and having a

"write" attribute (copy/retain/etc.). But, property declaration in 
primary class and protcols are tentative as they may be overridden 
into a 'readwrite' property in class extensions. Postpone diagnosing 
such warnings until the class implementation is seen. 
// rdar://11656982

llvm-svn: 158869