Decouple associated constants from let (#5973)

Decouples associated constants from being special cased in let handlers.
Enforces associated constant grammar restrictions in parsing instead of
checking.

Closes #5411

toolchain/parse/context.cpp

@@ -438,7 +438,8 @@ static auto ParsingInDeferredDefinitionScope(Context& context) -> bool {
   auto& stack = context.state_stack();
   if (stack.size() < 2 ||
       (stack.back().kind != StateKind::DeclScopeLoopAsClass &&
-       stack.back().kind != StateKind::DeclScopeLoopAsNonClass)) {
+       stack.back().kind != StateKind::DeclScopeLoopAsInterface &&
+       stack.back().kind != StateKind::DeclScopeLoopAsRegular)) {
     return false;
   }
   auto kind = stack[stack.size() - 2].kind;