Set a special flag in class metadata when an Objective-C class

Set a special flag in class metadata when an Objective-C class
has ivars that require destruction, but none that require anything
except zero-initialization.  This is common in ARC and (when true
throughout a class hierarchy) permits the elimination of an
unnecessary message-send during allocation.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@166088 91177308-0d34-0410-b5e6-96231b3b80d8