Split -Wunused-variable warning.

With r190382, -Wunused-variable warns about unused const variables when
appropriate. For codebases that use -Werror, this poses a problem as
existing unused const variables need to be cleaned up first. To make the
transistion easier, this patch splits -Wunused-variable by pulling out
an additional -Wunused-const-variable (by default activated along with
-Wunused-variable).