c++, coroutines: Allow NVRO in more cases for ramp functions.

c++, coroutines: Allow NVRO in more cases for ramp functions.

The constraints of the c++ coroutines specification require the ramp
to construct a return object early in the function.  This will be returned
at some later time.  This is implemented as NVRO but requires that copying
be well-formed even though it will be elided.  Special-case ramp functions
to allow this.

gcc/cp/ChangeLog:

	* typeck.cc (check_return_expr): Suppress conversions for NVRO
	in coroutine ramp functions.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
(cherry picked from commit d87caa9d3595ca845c9282cef8b0c9a656d8def0)