[analyzer] Detect ObjC properties that are both (copy) and Mutable.

When an Objective-C property has a (copy) attribute, the default setter
for this property performs a -copy on the object assigned.

Calling -copy on a mutable NS object such as NSMutableString etc.
produces an immutable object, NSString in our example.
Hence the getter becomes type-incorrect.

rdar://problem/21022397

Differential Revision: https://reviews.llvm.org/D27535

llvm-svn: 289554