This PR ultimately does 3 things
* Separately tracks matched tests per each filter part (that is, a set of filters separated by an OR (`,`)), which allows Catch2 to report each of the alternative filters that don't match any tests.
* Fixes `-w NoTests` to return non-zero in the process
* Adds tests for `-w NoTests`.
Swept:
`-Wpadded` in some places (where it caused extra size, instead of just
saying "hey, we padded struct at the end to align, just as standard says")
`-Wweak-vtables` everywhere (Clang)
`-Wexit-time-destructors` everywhere (Clang)
`-Wmissing-noreturn` everywhere (Clang)
The last three are enabled for Clang compilation going forward.
Also enabled `-Wunreachable-code` for Clang and GCC
