[Clang] Universal character names and Assertion failure in Clang 16

With Clang 16 (since https://reviews.llvm.org/D130750), the following code causes an assertion failure, if Clang is build with assertions ("works" if assertions are disabled). It used to work with Clang 15 (with and without assertions), so this seems to be a regression.

```c
void test() {
  int \u0024;
}
```

Tested on Linux with `clang -E <file.c>`