Refine -Wunused-variable to only suppress warning for __bridge_transfer, not all bridge casts.

Also refine test case to capture the intention of this suppression.  Essentially
some developers use __bridge_transfer as if it were a safe CFRelease.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194663 91177308-0d34-0410-b5e6-96231b3b80d8