Add a codegen test for the overwrite behavior

Author: mysterymath

clang/test/CodeGen/attr-modular-format.c

@@ -35,6 +35,15 @@ void test_order(void) {
   order2("hello");
 }
 
+int overwrite(const char *fmt, ...) __attribute__((modular_format(__impl1, "__name1", "1"), modular_format(__impl2, "__name2", "2"), format(printf, 1, 2)));
+
+// CHECK-LABEL: define dso_local void @test_overwrite(
+// CHECK:    {{.*}} = call i32 (ptr, ...) @overwrite(ptr noundef @.str) #[[ATTR_OVERWRITE:[0-9]+]]
+void test_overwrite(void) {
+  overwrite("hello");
+}
+
 // CHECK: attributes #[[ATTR]] = { "modular-format"="printf,1,2,__modular_printf,__printf,float" }
 // CHECK: attributes #[[ATTR_REDECL]] = { "modular-format"="printf,1,2,__second_impl,__second,three,two" }
 // CHECK: attributes #[[ATTR_ORDER]] = { "modular-format"="printf,1,2,__modular_printf,__printf,a,b" }
+// CHECK: attributes #[[ATTR_OVERWRITE]] = { "modular-format"="printf,1,2,__impl2,__name2,2" }