Previously we allowed these casts only for constants declared in system
headers, which we assume are retain/release-neutral. Now also allow them
for constants in user headers, treating them as +0.  Practically, this
means that we will now allow:
id x = (id)kMyGlobalConst;

But unlike with system headers we cannot mix them with +1 values:
id y = (id)(b ? kMyGlobalConst : [Obj newValAtPlusOne]); // error
id z = (id)(b ? kSystemGlobalConst: [Obj newValAtPlusOne]); // OK

Thanks to John for suggesting this improvement.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@230534 91177308-0d34-0410-b5e6-96231b3b80d8