Hide std::exception_ptr and friends if exceptions disabled

[ghost]

Description

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.

[codecov]

Codecov Report

Merging #1950 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1950   +/-   ##
=======================================
  Coverage   88.60%   88.60%           
=======================================
  Files         138      138           
  Lines        5616     5616           
=======================================
  Hits         4976     4976           
  Misses        640      640           

[horenmar]

I would prefer if all platforms provided stubbed out exception-related functions, but I do support -fno-except. Oh well.

Thanks.