Parse all kinds of declarations at function scope. (#4779)

These don't fully work in check and beyond yet, because they're not
added into lexical lookup, but already mostly do the right thing.

Per #3407, disallow namespace declarations anywhere other than at file
scope for now.

We don't treat statements starting with a packaging introducer keyword
(`package`, `library`, `import`) as declarations because they're
sufficiently unlikely to occur that the error recovery doesn't seem
important, and this avoids needing to disambiguate `package.` at the
start of an expression.

toolchain/parse/handle_decl_scope_loop.cpp

@@ -242,21 +242,8 @@ static auto TryHandleAsModifier(Context& context) -> bool {
   }
 }
 
-auto HandleDeclScopeLoop(Context& context) -> void {
-  // This maintains the current state unless we're at the end of the scope.
-
-  if (context.PositionIs(Lex::TokenKind::CloseCurlyBrace) ||
-      context.PositionIs(Lex::TokenKind::FileEnd)) {
-    // This is the end of the scope, so the loop state ends.
-    context.PopAndDiscardState();
-    return;
-  }
-
-  // Create a state with the correct starting position, with a dummy kind
-  // until we see the declaration's introducer.
-  Context::StateStackEntry state{.state = State::Invalid,
-                                 .token = *context.position(),
-                                 .subtree_start = context.tree().size()};
+auto HandleDecl(Context& context) -> void {
+  auto state = context.PopState();
 
   // Add a placeholder node, to be replaced by the declaration introducer once
   // it is found.
@@ -271,4 +258,16 @@ auto HandleDeclScopeLoop(Context& context) -> void {
   }
 }
 
+auto HandleDeclScopeLoop(Context& context) -> void {
+  // This maintains the current state unless we're at the end of the scope.
+  if (context.PositionIs(Lex::TokenKind::CloseCurlyBrace) ||
+      context.PositionIs(Lex::TokenKind::FileEnd)) {
+    // This is the end of the scope, so the loop state ends.
+    context.PopAndDiscardState();
+    return;
+  }
+
+  context.PushState(State::Decl);
+}
+
 }  // namespace Carbon::Parse