* Use Xenial as the base distribution
* Remove C++11 builds
* Add a lot more C++14 builds
* Add some C++17 builds
* Include newer versions of Clang and GCC
By using non-trivially copyable types, we force libstdc++-9's variant to
properly enter the valueless-by-exception state for our stringification
test.
Related to #1511
The StringMaker is off by default and can be enabled by a new macro `CATCH_CONFIG_ENABLE_VARIANT_STRINGMAKER`, to avoid increasing the footprint of stringification machinery by default.
