Support $foo::BAR where foo is declared as a string or a class-string

[spaze]

These should be supported

phpstan-disallowed-calls/tests/src/invalid/constantUsages.php

Lines 8 to 14 in 0c425fe

	/** @var string $monster */
	$monster = DateTime::class;
	$monster::COOKIE;
	/** @var class-string $monster */
	$monster = DateTime::class;
	$monster::COOKIE;

But currently they aren't:

phpstan-disallowed-calls/tests/ClassConstantInvalidUsagesTest.php

Lines 29 to 36 in 0c425fe

	[
	'Cannot access constant COOKIE on string',
	10,
	],
	[
	'Cannot access constant COOKIE on class-string',
	14,
	],

This is a follow-up to #42, which was fixed by #43 to just not crash.

[spaze]

Closing this now, eventually it seems that it's not a big deal in real world code, including mine. Can reopen if needed.