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_function.cpp

@@ -585,10 +585,9 @@ static auto HandleFunctionDefinitionAfterSignature(
   auto& function = context.functions().Get(function_id);
 
   // Create the function scope and the entry block.
-  context.return_scope_stack().push_back({.decl_id = decl_id});
+  context.scope_stack().PushForFunctionBody(decl_id);
   context.inst_block_stack().Push();
   context.region_stack().PushRegion(context.inst_block_stack().PeekOrAdd());
-  context.scope_stack().Push(decl_id);
   StartGenericDefinition(context);
 
   CheckFunctionDefinitionSignature(context, function);
@@ -645,9 +644,8 @@ auto HandleParseNode(Context& context, Parse::FunctionDefinitionId node_id)
     }
   }
 
-  context.scope_stack().Pop();
   context.inst_block_stack().Pop();
-  context.return_scope_stack().pop_back();
+  context.scope_stack().Pop();
   context.decl_name_stack().PopScope();
 
   auto& function = context.functions().Get(function_id);