Subzero: Enable errors for unused parameters.

Uses pragmas when including problematic LLVM headers, so that clang
doesn't warn about unused parameters -- a rampant problem in the LLVM
code.  This way, we can increase clang's pickiness on Subzero code,
and also not drown in LLVM warnings.

BUG= none
R=jpp@chromium.org, jvoung@chromium.org

Review URL: https://codereview.chromium.org/1217433002