meta/codegen.d: Fix getSymbols for dlang/dmd#11320

[ntrel]

This fixes #2455. Instead of changing the assert I just made getSymbols continue dropping qualifiers, because that will work with both existing and future dmd.

See https://github.com/dlang/dmd/pull/11320.

[Geod24]

A comment would be nice

[ntrel]

@geod24 I've edited the description and added links, hope that's ok

[schveiguy]

looking at the usage (figure out which imports need to happen to include these symbols), I think this change is correct in terms of what this thing is meant to do. I would suggest a document comment that clarifies that attributes will be stripped from types.