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/return.cpp

@@ -12,12 +12,11 @@ namespace Carbon::Check {
 
 // Gets the function that lexically encloses the current location.
 auto GetCurrentFunctionForReturn(Context& context) -> SemIR::Function& {
-  CARBON_CHECK(!context.return_scope_stack().empty(),
+  CARBON_CHECK(context.scope_stack().IsInFunctionScope(),
                "Handling return but not in a function");
-  auto function_id = context.insts()
-                         .GetAs<SemIR::FunctionDecl>(
-                             context.return_scope_stack().back().decl_id)
-                         .function_id;
+  auto decl_id = context.scope_stack().GetReturnScopeDeclId();
+  auto function_id =
+      context.insts().GetAs<SemIR::FunctionDecl>(decl_id).function_id;
   return context.functions().Get(function_id);
 }
 
@@ -33,9 +32,9 @@ auto GetCurrentReturnSlot(Context& context) -> SemIR::InstId {
 // Gets the currently in scope `returned var`, if any, that would be returned
 // by a `return var;`.
 static auto GetCurrentReturnedVar(Context& context) -> SemIR::InstId {
-  CARBON_CHECK(!context.return_scope_stack().empty(),
+  CARBON_CHECK(context.scope_stack().IsInFunctionScope(),
                "Handling return but not in a function");
-  return context.return_scope_stack().back().returned_var;
+  return context.scope_stack().GetReturnedVar();
 }
 
 // Produces a note that the given function has no explicit return type.