Constants are currently added to the scope if e.g. defined() is used and passed around, but never removed anymore. Which this PR changes.

The constants are added to the scope via

• a ConstFetch expression that is specified in https://github.com/phpstan/phpstan-src/blob/1.8.2/src/Type/Php/DefinedConstantTypeSpecifyingExtension.php#L55
• from now on we're in MutatingScope where either specifyExpressionType or removeTypeFromExpression is called depending on sure or sureNotType which is effectively depending on truthy/falsey context
• specifyExpressionType is adding the constants to the scope in https://github.com/phpstan/phpstan-src/blob/1.8.2/src/Analyser/MutatingScope.php#L3434-L3458
• removeTypeFromExpression is calling specifyExpressionType again with the removed type (in the constant example with e.g. !defined() leading to a NeverType)
• the constant was previously simply set again, now it is properly removed

(Specified)Type-wise this doesn't change anything, but ConstantRule explicitly verifies if a constant is in the scope and not which type it has. And this rule is working a bit better now I think.