common/assert: Make the UNIMPLEMENTED macro properly assert

Currently, this was only performing a logging call, which doesn't
actually invoke any assertion behavior. This is unlike
UNIMPLEMENTED_MSG, which *does* assert.

This makes the expected behavior uniform across both macros.

src/common/assert.h

@@ -52,5 +52,5 @@ __declspec(noinline, noreturn)
 #define DEBUG_ASSERT_MSG(_a_, _desc_, ...)
 #endif
 
-#define UNIMPLEMENTED() LOG_CRITICAL(Debug, "Unimplemented code!")
+#define UNIMPLEMENTED() ASSERT_MSG(false, "Unimplemented code!")
 #define UNIMPLEMENTED_MSG(...) ASSERT_MSG(false, __VA_ARGS__)