coroutines: Accept 'extern "C"' coroutines.

coroutines: Accept 'extern "C"' coroutines.

'extern "C"' coroutines are permitted by the standard and expected to work
(although constructing useful cases could be challenging). In order to
permit this we need to arrange for the outlined helper functions to be
named properly, even when no mangling is required.  To do this, we append
the actor and destroy suffixes in all cases.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
gcc/cp/ChangeLog:

	* mangle.cc (write_mangled_name): Append the helper function
	suffixes here...
	(write_encoding): ... rather than here.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/torture/extern-c-coroutine.C: New test.