Subzero: Try to fix warnings and errors in the Windows build.

Quiet some unused-variable warnings when their only use is in an assert().

Forward-declare partial template specializations when the template method already has a default implementation, to avoid ODR violations and link errors.

BUG= https://codereview.chromium.org/296053008/
R=wala@chromium.org

Review URL: https://codereview.chromium.org/429993002
3 files changed