phpstan version: 0.11.2

Summary of a problem or a feature request

Multiple DateTime and DateTimeImmutable methods like modify, diff, format, getOffset, getTimezone and setTime can return false on failure.

Code snippet that reproduces the problem

Example:

$validTo = new \DateTimeImmutable();
$validTo = $validTo->setTime(23, 59, 59);

// the problem is here
if ($validTo === false) {
    throw new RuntimeException(__('Invalid date'));
}

The output:

Strict comparison using === between DateTimeImmutable and false will  always evaluate to false.                     

Expected output

No error, because the the PHP documentation says:

https://secure.php.net/manual/en/datetime.settime.php#refsect1-datetime.settime-returnvalues

Returns the DateTime object for method chaining or FALSE on failure.

The same kind of error applies to multiple other DateTime and DateTimeImmutable methods like modify, diff, format, getOffset, getTimezone.