c-family: Implicitly return zero from main even on freestanding

... unless marked noreturn.

This should not get in anyone's way, but should permit the use of main()
in freestanding more easily, especially for writing test cases that
should work both in freestanding and hosted modes.

gcc/c/ChangeLog:

	* c-decl.cc (finish_function): Ignore hosted when deciding
	whether to implicitly return zero, but check noreturn.
	* c-objc-common.cc (c_missing_noreturn_ok_p): Loosen the
	requirements to just MAIN_NAME_P when hosted, or `int main'
	otherwise.

gcc/cp/ChangeLog:

	* cp-tree.h (DECL_MAIN_P): Move most logic, besides the hosted
	check, from here...
	(DECL_MAIN_ANY_P): ... to here, so that it can be reused ...
	(DECL_MAIN_FREESTANDING_P): ... here, with an additional
	constraint on (hosted OR return type == int)
	* decl.cc (finish_function): Use DECL_MAIN_FREESTANDING_P
	instead of DECL_MAIN_P, to loosen the hosted requirement, but
	check noreturn, before adding implicit returns.

gcc/testsuite/ChangeLog:

	* gcc.dg/noreturn-4.c: Removed.
	* g++.dg/freestanding-main.C: New test.
	* g++.dg/freestanding-nonint-main.C: New test.
	* gcc.dg/freestanding-main.c: New test.
	* gcc.dg/freestanding-nonint-main.c: New test.