Remove some unused poly_int variables

Remove some unused poly_int variables

Switching to default constructors for poly_int exposed some
unused variables that weren't previously diagnosed.

gcc/
	* dwarf2out.cc (mem_loc_descriptor): Remove unused variables.
	* tree-affine.cc (expr_to_aff_combination): Likewise.

gcc/cp/
	* constexpr.cc (cxx_fold_indirect_ref): Remove unused variables.

gcc/rust/
	* backend/rust-constexpr.cc (rs_fold_indirect_ref): Remove unused
	variables.