c++: remove coroutines.exp

c++: remove coroutines.exp

coroutines.exp was basically only there to add -std=c++20 to all the tests;
removing it lets us use the general support for running tests under multiple
standards.  Doing this revealed that some tests that specifically run in
C++17 mode were relying on -std=c++20 followed by -std=c++17 leaving
flag_coroutines set, which seems unintentional, and different from how we
handle other feature flags.  So this changes that, and adds the missing
-fcoroutines to those tests.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/co-await-syntax-09-convert.C: Add -fcoroutines.
	* g++.dg/coroutines/co-await-syntax-10.C
	* g++.dg/coroutines/co-await-syntax-11.C
	* g++.dg/coroutines/co-await-void_type.C
	* g++.dg/coroutines/co-return-warning-1.C
	* g++.dg/coroutines/ramp-return-a.C
	* g++.dg/coroutines/ramp-return-c.C: Likewise.
	* g++.dg/coroutines/coroutines.exp: Removed.
	* lib/g++-dg.exp: Start at C++20 for coroutines/

gcc/c-family/ChangeLog:

	* c-opts.cc (c_common_post_options): Set flag_coroutines.
	(set_std_cxx20, set_std_cxx23, set_std_cxx26): Not here.