Warn about more than the first unused variable when -Werror is set.

To do this, thread DiagnosticErrorTrap's hasUnrecoverableErrorOccurred through
to Scope.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@178294 91177308-0d34-0410-b5e6-96231b3b80d8