Implement std::any.

Implement std::any.

	* include/Makefile.am: Add any and c++17_warning.h to exported headers.
	* include/Makefile.in: Likewise.
	* include/std/any: New.
	* testsuite/20_util/any/assign/1.cc: Likewise.
	* testsuite/20_util/any/assign/2.cc: Likewise.
	* testsuite/20_util/any/assign/self.cc: Likewise.
	* testsuite/20_util/any/cons/1.cc: Likewise.
	* testsuite/20_util/any/cons/2.cc: Likewise.
	* testsuite/20_util/any/cons/aligned.cc: Likewise.
	* testsuite/20_util/any/cons/nontrivial.cc: Likewise.
	* testsuite/20_util/any/misc/any_cast.cc: Likewise.
	* testsuite/20_util/any/misc/any_cast_neg.cc: Likewise.
	* testsuite/20_util/any/misc/any_cast_no_rtti.cc: Likewise.
	* testsuite/20_util/any/misc/swap.cc: Likewise.
	* testsuite/20_util/any/modifiers/1.cc: Likewise.
	* testsuite/20_util/any/observers/type.cc: Likewise.
	* testsuite/20_util/any/typedefs.cc: Likewise.

From-SVN: r238061