[clang] Add test for CWG2486 (`noexcept` and function pointer conversion) (#107131)

[CWG2486](https://cplusplus.github.io/CWG/issues/2486.html) "Call to
`noexcept` function via `noexcept(false)` pointer/lvalue" allows
`noexcept` functions to be called via `noexcept(false)` pointers or
values. There appears to be no implementation divergence whatsoever:
https://godbolt.org/z/3afTfeEM8. That said, in C++14 and earlier we do
not issue all the diagnostics we issue in C++17 and newer, so I'm
specifying the status of the issue accordingly.