Detect control flow in entities nested inside functions (#5336)

Right now, return_scope_stack is being used to determine whether logic
is in a function scope. However, we need to handle nested entities
inside function scopes. For example where this crashes right now:

```
base class C(B:! bool) {}

fn F() {
  class B {
    extend base: C(true or false);
  }
}
```

This is doing a few things to make this kind of code not crash:

- Split `scope_stack().Push` into `PushForDeclName`, `PushForEntity`,
`PushForExpr`, and `PushForFunction` so that better decisions can be
made about behaviors.
- Hide `return_scope_stack` in the API, instead using interfaces to get
at the underlying data.
- Also using `PushForFunction` to update it similar to the other stacks
that `ScopeStack` manages.
- Add `IsInFunctionScope` as the best way to determine presence in
function scope.
- Remove `PeekIsLexicalScope` since destruction really wants function
scope information anyways.
- Clean up `destroy_id_stack` handling to be for function scopes rather
than lexical scopes.
- Return after related `context.TODO`s in a couple more spots, so that
code doesn't proceed to add control flow in spite of the lack of
support.

---------

Co-authored-by: Geoff Romer <gromer@google.com>

toolchain/check/handle_interface.cpp

@@ -193,8 +193,8 @@ auto HandleParseNode(Context& context,
                                         interface_info.self_param_id);
 
   // Enter the interface scope.
-  context.scope_stack().Push(interface_decl_id, interface_info.scope_id,
-                             self_specific_id);
+  context.scope_stack().PushForEntity(
+      interface_decl_id, interface_info.scope_id, self_specific_id);
 
   // TODO: Handle the case where there's control flow in the interface body. For
   // example: