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
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 should improve the compilation times by decreasing the number
of TUs compiled, without making overly big TUs that would cause
problems with heavy-tailed compilation times.
There is one "combined TU" for the top level part, and each subpart,
except for Reporters, which currently do not have any trivial TUs.
