Don't actually add the __unsafe_unretained qualifier in MRC;

Don't actually add the __unsafe_unretained qualifier in MRC;
driving a canonical difference between that and an unqualified
type is a really bad idea when both are valid.  Instead, remember
that it was there in a non-canonical way, then look for that in
the one place we really care about it: block captures.  The net
effect closely resembles the behavior of a decl attribute, except
still closely following ARC's standard qualifier parsing rules.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@253534 91177308-0d34-0410-b5e6-96231b3b80d8