Add signature enforcement for Main.Run and give it the symbol name main. (#3266)

For #2550.

toolchain/check/handle_function.cpp

@@ -4,6 +4,7 @@
 
 #include "toolchain/check/context.h"
 #include "toolchain/check/convert.h"
+#include "toolchain/sem_ir/entry_point.h"
 
 namespace Carbon::Check {
 
@@ -59,6 +60,21 @@ static auto BuildFunctionDeclaration(Context& context)
   auto decl_id = context.AddNode(
       SemIR::Node::FunctionDeclaration::Make(fn_node, function_id));
   context.declaration_name_stack().AddNameToLookup(name_context, decl_id);
+
+  if (SemIR::IsEntryPoint(context.semantics_ir(), function_id)) {
+    // TODO: Update this once valid signatures for the entry point are decided.
+    if (!context.semantics_ir().GetNodeBlock(param_refs_id).empty() ||
+        (return_slot_id.is_valid() &&
+         return_type_id !=
+             context.CanonicalizeType(SemIR::NodeId::BuiltinBoolType) &&
+         return_type_id != context.CanonicalizeTupleType(fn_node, {}))) {
+      CARBON_DIAGNOSTIC(InvalidMainRunSignature, Error,
+                        "Invalid signature for `Main.Run` function. Expected "
+                        "`fn ()` or `fn () -> i32`.");
+      context.emitter().Emit(fn_node, InvalidMainRunSignature);
+    }
+  }
+
   return {function_id, decl_id};
 }