* include/Makefile.am: Add new file std/variant.
	* include/Makefile.in: Generated from Makefile.am.
	* include/bits/enable_special_members.h: Add a tag type to allow
	the construction in non-default constructor.
	* include/bits/uses_allocator.h: Add convenience traits to
	detect constructibility.
	* include/std/variant: Implement <variant>.
	* testsuite/20_util/variant/compile.cc: Compile-time tests.
	* testsuite/20_util/variant/run.cc: Runtime tests.

From-SVN: r239590