Added a test.

Author: kstoimenov

compiler-rt/test/ubsan_minimal/TestCases/override-callback.c

@@ -0,0 +1,15 @@
+// RUN: %clang -fsanitize=implicit-integer-sign-change %s -o %t && %run %t 0 2>&1 | FileCheck %s
+#include <stdio.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+static int Result;
+
+void __ubsan_report_error(const char *msg, uintptr_t caller, int abort) {
+  fprintf(stderr, "CUSTOM_CALLBACK: %s %d\n", msg, abort);
+}
+
+int main(int argc, const char** argv) {
+  int32_t t0 = (~((uint32_t)0));
+// CHECK: CUSTOM_CALLBACK: implicit-conversion 0
+}