Add variant_gcc_false_positive.cpp to reproduce GCC 12+ spurious
-Wmaybe-uninitialized warnings when copying/moving variant containing
non-trivially-copyable types like std::exception_ptr and std::string.
Tests use boost::system::result which internally uses variant2, and
include coroutine-based tests when C++20 coroutine support is available.
The test uses -O3 optimization to trigger the aggressive inlining that
exposes the false positive where GCC's dataflow analysis cannot prove
the discriminator ensures only initialized alternatives are accessed.
See https://github.com/boostorg/variant2/issues/33
