[clang] Move CWG2390 test into `cwg23xx.cpp` (#94206)

This patch refactors an existing test for
[CWG2390](https://cplusplus.github.io/CWG/issues/2390.html) "Is the
argument of `__has_cpp_attribute` macro-expanded?" to use `#error`
instead of emitting a variable in IRGen and checking it via FileCheck.
As a bonus, this makes it possible to move the test into `cwg23xx.cpp`.