Improve pretty printer for std::variant

Improve pretty printer for std::variant

	* python/libstdcxx/v6/printers.py (SingleObjContainerPrinter): Allow
	display_hint to be set by subclasses.
	(StdVariantPrinter): Use array for display_hint. Adjust output to be
	more similar to std::any and std::optional output.
	(register_type_printers): Add type printers for basic_string_view
	typedefs and experimental::any. Adjust type printers for
	fundamentals_v1 templates to match fundamentals_v2 and later.
	* testsuite/libstdc++-prettyprinters/cxx17.cc: New.

From-SVN: r240234