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
diff --git a/src/IceTimerTree.cpp b/src/IceTimerTree.cpp
index de84ea5..879f06e 100644
--- a/src/IceTimerTree.cpp
+++ b/src/IceTimerTree.cpp
@@ -15,7 +15,11 @@
 #include "IceTimerTree.h"
 
 #include "IceDefs.h"
+
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunused-parameter"
 #include "llvm/Support/Timer.h"
+#pragma clang diagnostic pop
 
 namespace Ice {