Enable compiler errors for implicit switch case fallthroughs.

Also unify warning options across Clang and GCC and fix minor issues that was surfaced by this.

PiperOrigin-RevId: 508619655
Change-Id: I59b777bf2dfea4422485670e2427c7d3f5cf6405

test/xml/xml_native_writer_test.cc

@@ -917,8 +917,8 @@ TEST_F(XMLWriterLocaleTest, IgnoresLocale) {
   mj_deleteModel(model);
 
   // Test that MuJoCo doesn't override locales for subsequent calls.
-  char formatted[7];
-  std::snprintf(formatted, sizeof(formatted), "%f", 3.9375);
+  char formatted[8];
+  std::snprintf(formatted, sizeof(formatted), "%.4f", 3.9375);
   EXPECT_EQ(std::string(formatted), "3,9375");
 }