ICE for user-defined value type in external library function

## Description

The following code triggers the ICE shown below in the latest solc version ```0.8.30```. The issue appears to have started with version `0.8.8`, which was when user-defined value types were introduced.

## Steps to Reproduce

`testcase.sol`

```solidity
// SPDX-License-Identifier: GPL-3.0

pragma solidity =0.8.30;

type T is bool;

library L {
function f(T[] storage p) external {
}
}
```

`
solc testcase.sol
`

```
Internal compiler error:
/solidity/libsolidity/ast/Types.h(1199): Throw in function virtual std::string solidity::frontend::UserDefinedValueType::signatureInExternalFunction(bool) const
Dynamic exception type: boost::wrapexcept
std::exception::what: Solidity assertion failed
[solidity::util::tag_comment*] = Solidity assertion failed
```

## Environment

- Compiler version: ```0.8.30```

Start with testcase.sol and reproduce the failure using solc 0.8.30. Then inspect the UserDefinedValueType::signatureInExternalFunction assertion reported at Types.h(1199), including how the external library function handles T[] storage. Done means this input no longer triggers an internal compiler error and receives appropriate compiler handling.

Written by the indexing model from the issue text.