The problem was that under specific circumstances, namely that none
of their children progressed, `GeneratorTracker` will not progress.
This was changed recently, to allow for code like this, where a
`SECTION` follows a `GENERATE` at the same level:

```cpp
SECTION("A") {}
auto a = GENERATE(1, 2);
SECTION("B") {}
```

However, this interacted badly with `SECTION` filters (`-c foo`),
as they could deactivate all `SECTION`s below a generator, and thus
stop it from progressing forever. This commit makes GeneratorTracker
check whether there are any filters active, and if they are, it checks
whether its section-children can ever run.

Fixes #2025

The problem was that Catch2 did not reliably include `<exception>`
before it checked for the feature test macro for
`std::uncaught_exceptions`. To avoid overhead of including
`<exception>` everywhere, the configuration check was split out
into a separate header.

Closes #2021

As far as I understand the standard, if there is a function called
`rng` in the global namespace, and a function argument called `rng`,
then the argument should shadow the function. This then means that
uses of `rng` inside the function should refer to the argument.

This is not the case for AppleClang 12.0.0. Luckily the workaround
is simple enough; just rename the argument. Given that the function
is 3 lines and uncomplicated, the change of the name doesn't really
affect readability.

Still, WTF AppleClang?

Closes #2030