Objective-C. Do not issue warning when 'readonly'

Objective-C. Do not issue warning when 'readonly'
property declaration has a memory management
attribute (retain, copy, etc.). Sich properties
are usually overridden to become 'readwrite'
via a class extension (which require the memory
management attribute specified). In the absence of class
extension override, memory management attribute is
needed to produce correct Code Gen. for the
property getter in any case and this warning becomes
confusing to user. // rdar://15641300

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197251 91177308-0d34-0410-b5e6-96231b3b80d8