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/decl_name_stack.cpp

@@ -62,7 +62,7 @@ auto DeclNameStack::PushScopeAndStartName() -> void {
   decl_name_stack_.push_back(MakeEmptyNameContext());
 
   // Create a scope for any parameters introduced in this name.
-  context_->scope_stack().Push();
+  context_->scope_stack().PushForDeclName();
 }
 
 auto DeclNameStack::FinishName(const NameComponent& name) -> NameContext {
@@ -233,8 +233,8 @@ static auto PushNameQualifierScope(Context& context, SemIR::LocId loc_id,
   // providing the definition.
   StartGenericDecl(context);
 
-  context.scope_stack().Push(scope_inst_id, scope_id, self_specific_id,
-                             has_error);
+  context.scope_stack().PushForEntity(scope_inst_id, scope_id, self_specific_id,
+                                      has_error);
 
   // An interface also introduces its 'Self' parameter into scope, despite it
   // not being redeclared as part of the qualifier.
@@ -246,7 +246,7 @@ static auto PushNameQualifierScope(Context& context, SemIR::LocId loc_id,
   }
 
   // Enter a parameter scope in case the qualified name itself has parameters.
-  context.scope_stack().Push();
+  context.scope_stack().PushForSameRegion();
 }
 
 auto DeclNameStack::ApplyNameQualifier(const NameComponent& name) -> void {