Add -Wobjc-property-assign-on-object-type.

This is a warning about using 'assign' instead of 'unsafe_unretained'
in Objective-C property declarations.  It's off by default because there
isn't consensus in the Objective-C steering group that this is the right
thing to do, but we're nonetheless okay with adding it because there's a
substantial pool of Objective-C programmers who will appreciate the warning.

Patch by Alfred Zien!

llvm-svn: 341489