c++: Replace "C++2a" with "C++20".

C++20 isn't final quite yet, but all that remains is formalities, so let's
go ahead and change all the references.

I think for the next C++ standard we can just call it C++23 rather than
C++2b, since the committee has been consistent about time-based releases
rather than feature-based.

gcc/c-family/ChangeLog
2020-05-13  Jason Merrill  <jason@redhat.com>

	* c.opt (std=c++20): Make c++2a the alias.
	(std=gnu++20): Likewise.
	* c-common.h (cxx_dialect): Change cxx2a to cxx20.
	* c-opts.c: Adjust.
	* c-cppbuiltin.c: Adjust.
	* c-ubsan.c: Adjust.
	* c-warn.c: Adjust.

gcc/cp/ChangeLog
2020-05-13  Jason Merrill  <jason@redhat.com>

	* call.c, class.c, constexpr.c, constraint.cc, decl.c, init.c,
	lambda.c, lex.c, method.c, name-lookup.c, parser.c, pt.c, tree.c,
	typeck2.c: Change cxx2a to cxx20.

libcpp/ChangeLog
2020-05-13  Jason Merrill  <jason@redhat.com>

	* include/cpplib.h (enum c_lang): Change CXX2A to CXX20.
	* init.c, lex.c: Adjust.