c++: Add -std=c++20.

c++: Add -std=c++20.

It's probably past time for this, but definitely now that we're done with
the final committee meeting of C++20.  This patch only adds the option and
adjusts the testsuite to recognize it; more extensive changes can wait for
the published standard.

gcc/ChangeLog
2020-02-15  Jason Merrill  <jason@redhat.com>

	* doc/invoke.texi (C Dialect Options): Add -std=c++20.

gcc/c-family/ChangeLog
2020-02-15  Jason Merrill  <jason@redhat.com>

	* c.opt: Add -std=c++20.

gcc/testsuite/ChangeLog
2020-02-15  Jason Merrill  <jason@redhat.com>

	* lib/target-supports.exp (check_effective_target_c++2a_only): Also
	look for -std=*++20.
	(check_effective_target_concepts): Use check_effective_target_c++2a.