Some compilers, e.g. the Green Hills C++ compiler, react badly to the
appearance of std::exception_ptr, std::current_exception,
std::rethrow_exception and std::uncaught_exception(s). To allow usage of
Catch2 with these compilers when exceptions are disabled, hide the usage
of std::exception_ptr etc. when compiling with
CATCH_CONFIG_DISABLE_EXCEPTIONS.
This means
* Adding new configuration toggle `CATCH_CONFIG_DISABLE_EXCEPTIONS`
and a best-guess configuration auto-checking for it.
* Adding new set of internal macros, `CATCH_TRY`, `CATCH_CATCH_ALL`
and `CATCH_CATCH_ANON` that can be used in place of regular `try`,
`catch(...)` and `catch(T const&)` respectively, while disappearing
when `CATCH_CONFIG_DISABLE_EXCEPTIONS` is enabled.
* Replacing all uses of `throw` with calls to `Catch::throw_exception`
customization point.
* Providing a default implementation for the above customization point
when `CATCH_CONFIG_DISABLE_EXCEPTIONS` is set.
* Letting users override this implementation with their own.
* Some minor changes and ifdefs all around to support the above
