Subzero: Use more "= default;" for ctors and dtors.

Look for "() override {}" and "() final {}" patterns.

Don't touch IceTargetLoweringX8632.* to spare a refactoring in
progress.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1201023002
diff --git a/src/IceCompileServer.h b/src/IceCompileServer.h
index 497810a..b5093df 100644
--- a/src/IceCompileServer.h
+++ b/src/IceCompileServer.h
@@ -69,7 +69,7 @@
   CLCompileServer(Compiler &Comp, int argc, char **argv)
       : CompileServer(Comp), argc(argc), argv(argv) {}
 
-  ~CLCompileServer() final {}
+  ~CLCompileServer() final = default;
 
   void run() final;