`GeneralizingAstVisitor` now triggers `NamedCompilationUnitMember` for all `FunctionDeclaration`s

[FMorschel]

This recent change https://dart-review.googlesource.com/c/sdk/+/442561 broke this old change f61adaf that made FunctionDeclaration not trigger NamedCompilationUnitMember visit in GeneralizingAstVisitor.

@bwilkerson:

The more appropriate thing to have done would be to either:

• Have a TopLevelFunctionDeclaration that wrapped a FunctionDeclaration the way a FunctionDeclarationStatement does, or
• Use a technique other than wrapping to share the API of FunctionDeclaration between those two classes.

Having a separate TopLevelFunctionDeclaration that implements NamedCompilationUnitMember would have made the intended behavior of the visitor more clear. That hacky, and in some ways counter intuitive, check was there to cover over the mistake in the design of the ASTNode hierarchy. [... The change in f61adaf] needs to be added back in.

We should probably add some tests to make sure this doesn't regress too (although the changes I'm making towards #60321 should cover that too; they will need this fix to land first).

FYI @fshcheglov, @scheglov, @DanTup

[scheglov]

Indeed, this is a regression.
We will fix this by re-introducing special casing for now, and later see how to make the hierarchies clean.

@fshcheglov Could you please fix the generator?

[fshcheglov]

https://dart-review.googlesource.com/c/sdk/+/443181