Objective-C properties: loosen 'atomic' checking for readonly properties.

r251874 reworked the way we handle properties declared within
Objective-C class extensions, which had the effective of tightening up
property checking in a number of places. In this particular class of
cases, we end up complaining about "atomic" mismatches between an
implicitly-atomic, readonly property and a nonatomic, readwrite
property, which doesn't make sense because "atomic" is essentially
irrelevant to readonly properties.

Therefore, suppress this diagnostic when the readonly property is
implicitly atomic. Fixes rdar://problem/23803109.

llvm-svn: 255174