allow them to be written in certain kinds of user declaration and
diagnose on the use-site instead.

Also, improve and fix some diagnostics relating to __weak and
properties.

rdar://23228631

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@251384 91177308-0d34-0410-b5e6-96231b3b80d8