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/parse.cpp

@@ -31,7 +31,7 @@ auto Parse(Lex::TokenizedBuffer& tokens, ParseOptions options) -> Tree {
   context.AddLeafNode(NodeKind::FileStart,
                       context.ConsumeChecked(Lex::TokenKind::FileStart));
 
-  context.PushState(StateKind::DeclScopeLoopAsNonClass);
+  context.PushState(StateKind::DeclScopeLoopAsRegular);
 
   while (!context.state_stack().empty()) {
     switch (context.state_stack().back().kind) {