Disallow qualified types in base class list

[Bolpat]

Currently, it is allowed that a base class is a qualified type.

class C : const(Object) { }
auto obj = new class const(Object) { };

The qualifier is ignored. For that reason, this change makes it a parse error, so no code fools a reader into thinking the const does something. Semantics are unaffected, i.e. if a typeof type is used which ends up being a qualified class type, the unqualified version of that class type remains a base class (no error).

The assumption of this PR is that no-one actually has qualified base classes in their code base (because the qualifier does nothing but mislead the reader).

It is in the interest of the Primary Type Syntax DIP and possibly future tuple DIPs to avoid parsing ambiguities. This can be done by requiring the base class list to be lexically comprised of new-terminology basic types. Refer to the accompanying spec PR for details.

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#16882"

[thewilsonator]

Does this fix an issue? if so please retitle the commit message to fix it, otherwise please add a changelog entry.