[Basic] Fix ~Diagnostic() to not call Emit() when there is no diag obj (most

commonly when the diag has been copied).
 - This eliminates a completely spurious call to Emit() which was in
   Sema::Diag(). Sob.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@152640 91177308-0d34-0410-b5e6-96231b3b80d8