### Godot version

v4.0.rc1.official [8843d9ad3]

### System information

Lubuntu

### Issue description

You can't use + or += to mutate typed arrays.

It seems to be not correctly getting the type information from the second operand.

An erudite poster on the discord puts it differently:
> it's more like operator + doesn't retain Array typing
> the syntax for Array Concatenation via operators, but the idea gets across i think


The error you see is:
```Trying to assign an array of type "Array" to a variable of type "Array[int]".```


### Steps to reproduce

```
	var Alice: Array[int] = [1,2,3]
	var Bob: Array[int] = [4,5,6]
	Alice += Bob
```
This will crash with the above error message.

### Minimal reproduction project

[plus_syntax_broken_demo_project.zip](https://github.com/godotengine/godot/files/10694514/plus_syntax_broken_demo_project.zip)

break;
							
							case Variant::RID:
							case Variant::BOOL:
							case Variant::CALLABLE:
							case Variant::FLOAT:
							case Variant::INT:
							case Variant::NIL:
							case Variant::NODE_PATH:
							case Variant::SIGNAL:
							case Variant::STRING_NAME:
								break;
							
							case Variant::DICTIONARY:
							case Variant::VARIANT_MAX:
								break;
						}
						if (error) {
							push_error(vformat(R"(Invalid index type "%s" for a base of type "%s".)", index_type.to_string(), base_type.to_string()), p_subscript->index);
						}
					}
				} else if (base_type.kind != GDScriptParser::DataType::BUILTIN && !index_type.is_variant()) {
					if (index_type.builtin_type != Variant::STRING && index_type.builtin_type != Variant::STRING_NAME) {
						push_error(vformat(R"(Only String or StringName can be used as index for type "%s", but received a "%s".)", base_type.to_string(), index_type.to_string()), p_subscript->index);
					}
				}
				
				result_type.builtin_type = Variant::NIL;
				result_type.kind = GDScriptParser::DataType::BUILTIN;
				result_type.type_source = base_type.is_hard_type() ? GDScriptParser::DataType::ANNOTATED_INFERRED : GDScriptParser::DataType::INFERRED;