Add Unreachable.h

This is to indicate when a code path is absolutely, for certain, 100%
unreachable. Useful as the default case inside a case-switch that is for
sure 100% exhaustive because it's inside the case of another case-switch
(and the default case is there to suppress compiler warnings about the
case-switch not being exhaustive), which is a situation coming up in my
scriptclass::startgamemode refactor.

It does this by deliberately invoking undefined behavior, either using a
compiler builtin that does the same thing or being a noreturn function
that returns. (And undefined behavior is not undefined behavior if it is
not executed in a code path, otherwise all NULL checks would be useless
because it'd dereference something that could be NULL in another code
path.)

desktop_version/src/Unreachable.h

@@ -0,0 +1,17 @@
+#ifndef UNREACHABLE_H
+#define UNREACHABLE_H
+
+#include <SDL_stdinc.h>
+
+SDL_NORETURN SDL_INLINE void VVV_unreachable(void)
+{
+    /* __builtin_unreachable() and __assume(0) execute undefined behavior.
+     * Otherwise, a noreturn function returning is also undefined behavior. */
+#if defined(__has_builtin) && __has_builtin(__builtin_unreachable)
+    __builtin_unreachable();
+#elif defined(_MSC_VER)
+    __assume(0);
+#endif
+}
+
+#endif /* UNREACHABLE_H */