Turn off partitioning unless it was explicitly requested, as it doesn't
work with D exception chaining, where personality routines use LSDA to
determine whether two thrown exceptions are in the same context.

The following distills what was failing in the D testsuite.
```
try {
  try {
    fn();  // throws "1"
  }
  finally {
    throw new Exception("2");
  }
}
catch (Exception e) {
  assert(e.msg == "1");
  assert(e.next.msg == "2");
}
```

gcc/d/ChangeLog:

	PR d/87824
	* d-lang.cc (d_post_options): Disable implicit
	-forder-blocks-and-partition.

From-SVN: r267985