Improve the predicates getABaseType, getABaseTypeDecl, getADerivedType and getADerivedTypeDecl on Type and TypeDecl:

• they behave consistently now.
• they all explore through type aliases in the declared base type now.
  • i.e. from B to the underlying A in class B : A_alias
  • however they don't resolve a type alias they are called directly upon, i.e. TypeAlias.getABaseType() has no result now. Previously there was special code to make this work for Type (but not TypeDecl). See below * for more discussion of this point.
• they all explore through protocols added in extensions now.
• the TODO in TypeDecl.qll added by @redsun82 in Swift: rename base_types in TypeDecl to inherited_types #14208 is resolved.
• some QLDoc comments added.

* - regarding the old special case for TypeAlias.getABaseType(). We could restore the old behaviour (and extend it to TypeDecl), but I currently don't think it's the right thing to do and I want to discuss that (and perhaps be persuaded to change my mind). Consider:

typealias A_alias = A
class B : A_alias { }
typealias B_alias = B

As this PR stands now, getABaseType takes us from B to A and getADerivedType from A to B. Simple. It's the users job to call .getUnderlyingType if they might be working with a B_alias.

If we have getABaseType take us from B_alias to A as well (that is from both B and B_alias to A), then (assuming we keep the symmetry between them...) getADerivedType has two results for A, one of which is itself an alias. But then surely we want getADerivedType to work in the other direction from A_alias - implying additional results from getABaseType as well. That means both functions are many-to-many, and hard to think clearly about. Especially if there are more type aliases than in the example above...

As this PR stands I have removed the special case, updated the existing code that relied on it, and called it out explicitly in the change note.

There's a good chance this change will affect results on tests I haven't run locally. Once CI passes, I will do a DCA run as well.