ModelASTWalker was previously constructing SyntaxNodes for EnumElementDecls
manually when visiting their associated EnumCaseDecl so that they would appear
as children rather than siblings. It wasn't actually walking these nodes
though, so missed handling some things, e.g. closures passed as default
argument values. These were also still being visited later, and because the
first visit consumed all the associated TokenNodes, this was triggering an
assertion due to the associated TokenNodes not matching expectations.
- When parsing a type or extension declaration, attempt to parse a function or property declaration when meeting an identifier, an operator or a paren (for tuple declarations).
- Produce the diagnostic with a fix-it suggesting to insert the needed keyword
- Recover parsing as if the declaration with the missing keyword is a function/property declaration
Resolves https://bugs.swift.org/browse/SR-10477
