When running clang.exe under Windows, catch.hpp leaks warning
suppressions because it uses `#pragma warning(push)` & `#pragma
warning(pop)` around warning suppressions like `#pragma clang diagnostic
ignore "-Wunused-variable"`, instead of using `#pragma clang diagnostic
push` and `#pragma clang diagnostic pop`.

This fixes that by only defining
`CATCH_INTERNAL_START_WARNINGS_SUPPRESSION` and
`CATCH_INTERNAL_STOP_WARNINGS_SUPPRESSION` to be the cl.exe variants if
`defined(_MSC_VER) && !defined(__clang__)`.

This adds prefix, exec_prefix and libdir fields

* 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

Closes #2207

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();
                                         ^~~~~~

Such identifiers are reserved by the C++ standard.
Fixes part of #578.