Allow the results of cf_returns_not_retained function

Allow the results of cf_returns_not_retained function
calls, or calls to audited functions without an explicit
return attribute, to be casted without a bridge cast.
Tie this mechanism in with the existing exceptions to
the cast restrictions.  State those restrictions more
correctly and generalize.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@140912 91177308-0d34-0410-b5e6-96231b3b80d8