Track type qualifiers separately

Using a special qualified type kind has a number of problems:
- Important fields such as size, align, and incomplete may not be set,
  since the qualified type was created before a struct was completed.
- When we don't care about type qualifiers (which is the usual case),
  we have to explicitly unqualify the type which is annoying and
  error-prone.

Instead, in derived types, keep track of the qualifiers of the base type
alongside the base type (similar to what is done for members, parameters,
declarations, and expressions in the past few commits).

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