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/control_flow.cpp

@@ -139,8 +139,8 @@ auto IsCurrentPositionReachable(Context& context) -> bool {
 
 auto MaybeAddCleanupForInst(Context& context, SemIR::TypeId type_id,
                             SemIR::InstId inst_id) -> void {
-  if (!context.scope_stack().PeekIsLexicalScope()) {
-    // Cleanup can only occur in lexical scopes.
+  if (!context.scope_stack().IsInFunctionScope()) {
+    // Cleanup can only occur in function scopes.
     return;
   }