Implement the 'null_resettable' attribute for Objective-C properties.

'null_resettable' properties are those whose getters return nonnull
but whose setters take nil, to "reset" the property to some
default. Implements rdar://problem/19051334.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240155 91177308-0d34-0410-b5e6-96231b3b80d8