objective-C: Fixes a bogus warning due to not setting
the "nonatomic" attribute in property redeclaration
in class extension. Also, improved on diagnostics in
this area while at it. // rdar://13156292


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174821 91177308-0d34-0410-b5e6-96231b3b80d8