[arcmt] In GC, handle (assign) @properties.

[arcmt] In GC, handle (assign) @properties.

-Move __strong/__weak added to a property type to the property attribute,
e.g.  "@property (assign) __weak Foo *prop;" --> "@property (weak) Foo *prop;"

-Remove (assign) in a property so that it becomes strong-by-default in ARC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@143979 91177308-0d34-0410-b5e6-96231b3b80d8