This is necessary when we want to differentiate between type reference
on extension declaration's start, e.g "extension A {}", and other
references of "A". NFC on existing functionality.
Also rename ASTWalker::shouldWalkIntoFunctionGenericParams() to shouldWalkIntoGenericParams() since it's now used when walking NominalTypeDecl (not just AbstractFunctionDecl).
* [RangeInfo] Perform complete walk on the last node in a given range to avoid missing information due to early abort.
* [RangeInfo] For declared decls in a given range, report whether they are referenced outside of the range.
* [RangeInfo] Report the contained (topmost) ASTNodes inside a selected range.
