Tell Clang's static analysis that SDL_assert() is an assertion handler.

This lets it know, for example, that when you do this...

    SDL_assert(ptr != NULL);

...that (ptr) is definitely not NULL at this point in the program, for the
sake of static analysis. While a buggy program could definitely trigger this
assertion, Clang assumes your assertion check is covering it and won't
report possible NULL dereferences after this point.

Since SDL_assert might continue if the user clicks "ignore", without this
change Clang would notice you checked for NULL (meaning that NULL is a real
possibility here) and still wrote code outside of that test branch that
dereferences the pointer, and thus would always trigger false positives.

Static analysis is fun!

include/SDL_assert.h

@@ -120,7 +120,14 @@ typedef struct SDL_assert_data
 /* Never call this directly. Use the SDL_assert* macros. */
 extern DECLSPEC SDL_assert_state SDLCALL SDL_ReportAssertion(SDL_assert_data *,
                                                              const char *,
-                                                             const char *, int);
+                                                             const char *, int)
+#if defined(__clang__) && __has_feature(attribute_analyzer_noreturn)
+/* this tells Clang's static analysis that we're a custom assert function,
+   and that the analyzer should assume the condition was always true past this
+   SDL_assert test. */
+   __attribute__((analyzer_noreturn))
+#endif
+;
 
 /* the do {} while(0) avoids dangling else problems:
     if (x) SDL_assert(y); else blah();