Warn on unused auto variables.

To do so we have to wait until we know that the type of a variable has been
deduced. Sema::FinalizeDeclaration is the first callback that is used for
decl with or without initializers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171458 91177308-0d34-0410-b5e6-96231b3b80d8