AstGen: add error for named function type

Closes #12660

src/AstGen.zig

@@ -1164,6 +1164,10 @@ fn fnProtoExpr(
     const tree = astgen.tree;
     const token_tags = tree.tokens.items(.tag);
 
+    if (fn_proto.name_token) |some| {
+        return astgen.failTok(some, "function type cannot have a name", .{});
+    }
+
     const is_extern = blk: {
         const maybe_extern_token = fn_proto.extern_export_inline_token orelse break :blk false;
         break :blk token_tags[maybe_extern_token] == .keyword_extern;

test/cases/compile_errors/function_type_named.zig

@@ -0,0 +1,7 @@
+const aFunc = fn someFunc(x: i32) void;
+
+// error
+// backend=stage2
+// target=native
+//
+// :1:18: error: function type cannot have a name