Swift: Add change note.

Author: geoffw0

swift/ql/lib/change-notes/2023-09-18-get-a-base-type.md

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+---
+category: majorAnalysis
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+* The predicates `getABaseType`, `getABaseTypeDecl`, `getADerivedType` and `getADerivedTypeDecl` on `Type` and `TypeDecl` now behave more usefully and consistently. They now explore through type aliases used in base class declarations, and include protocols added in extensions.
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+To examine base class declarations at a low level without these enhancements, use `TypeDecl.getInheritedType`.
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+`Type.getABaseType` (only) previously resolved a type alias it was called directly on. This behaviour no longer exists. To find any base type of a type that could be an alias, the construct `Type.getUnderlyingType().getABaseType*()` is recommended.