[enh] Issue 8765 - Assert should print expression when no message is present

This makes this code:

```d
assert(1 == 2);
```

Print:

```core.exception.AssertError@test.d(2): assert(1 == 2)```

But I can't figure out a way to write a test-case because the test is compilable but it's expected to fail at runtime. I need to compare the runtime message just like diag-style tests in the **fail_compilation** folder. Any help there is appreciated.