Made function and namespace nodes typed to remove a crash (#3285)

Bug found by fuzzing. Problem was untyped SemIR nodes had an invalid
type id, which was retrieved by `HandlePrefixOperator` and then passed
to `context.GetUnqualifiedType`, ultimately performing an invalid access
in `semantics_ir_->GetNode`.

We prefer to make a placeholder type for functions and namespaces to
remove the need for checking for the untyped case everywhere. Eventually
functions will have their own types, but this approach will be needed
for namespaces (and perhaps other non-first-class entities like unbound
methods and interface members) long term.

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/handle_function.cpp

@@ -57,8 +57,9 @@ static auto BuildFunctionDeclaration(Context& context)
        .return_type_id = return_type_id,
        .return_slot_id = return_slot_id,
        .body_block_ids = {}});
-  auto decl_id =
-      context.AddNode(SemIR::FunctionDeclaration(fn_node, function_id));
+  auto decl_id = context.AddNode(SemIR::FunctionDeclaration(
+      fn_node, context.CanonicalizeType(SemIR::NodeId::BuiltinFunctionType),
+      function_id));
   context.declaration_name_stack().AddNameToLookup(name_context, decl_id);
 
   if (SemIR::IsEntryPoint(context.semantics_ir(), function_id)) {