c++: Fix coroutines on targetm.cxx.cdtor_return_this targets [PR99459]

c++: Fix coroutines on targetm.cxx.cdtor_return_this targets [PR99459]

The r11-7528 build_co_await changes broke coroutines on arm*-linux-gnuabi,
2780 ^FAIL.*coroutines/ in total.
The problem is that arm is targetm.cxx.cdtor_return_this target where
both ctors and dtors in the ABI return this pointer rather than
void, and build_new_method_call_1 does:
              else if (call != error_mark_node
                       && DECL_DESTRUCTOR_P (cand->fn)
                       && !VOID_TYPE_P (TREE_TYPE (call)))
                /* An explicit call of the form "x->~X()" has type
                   "void".  However, on platforms where destructors
                   return "this" (i.e., those where
                   targetm.cxx.cdtor_returns_this is true), such calls
                   will appear to have a return value of pointer type
                   to the low-level call machinery.  We do not want to
                   change the low-level machinery, since we want to be
                   able to optimize "delete f()" on such platforms as
                   "operator delete(~X(f()))" (rather than generating
                   "t = f(), ~X(t), operator delete (t)").  */
                call = build_nop (void_type_node, call);
The new code in build_co_await relies on build_special_member_call
returned expression being a CALL_EXPR, but due to the build_nop
in there it is a NOP_EXPR around the CALL_EXPR.  It can't be stripped
with STRIP_NOPS because void has different mode from the pointer mode.

2021-03-09  Jakub Jelinek  <jakub@redhat.com>

	PR c++/99459
	* coroutines.cc (build_co_await): Look through NOP_EXPRs in
	build_special_member_call return value to find the CALL_EXPR.
	Simplify.