Remove the space between "" and the suffix in user-defined literal
operators (e.g. operator ""_a instead of operator "" _a) to avoid
MSVC deprecation warning C5311.

Fixes #2416

Refactor FilterGenerator to remove ctor call to overridden method next()
in order to address clang static analyzer diagnostic:

catch2-src/single_include/catch2/catch.hpp:4166:42: note: Call to virtual method 'FilterGenerator::next' during construction bypasses virtual dispatch
                auto has_initial_value = next();
                                         ^~~~~~

* Suppressed warning for comma-in-indexing-operator in tests that check
  that specific behaviour.
* Made deprecated (and removed) allocator usings conditional on the tests
  being compiled with old version of MSVC that still requires them.

Fixes #2272

Should fix #2116

This basically tests the combination where the compiler supports most
of C++17 but the library does not.

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