testsuite, c++, coroutines: Avoid 'unused' warnings [NFC].

The 'torture' section of the coroutine tests is primarily about checking
correct operation of the generated code.  It should, ideally, be possible
to run this part of the testsuite with '-Wall' and expect no fails.  In
the case that we wish to test for a specific diagnostic (and that it does
not appear over a range of optimisation/debug conditions) then we should
make that explict (as done, for example, in pr109867.C).

The tests amended here have warnings because of unused entities; in many
cases those are relevant to the test, and so we just mark them with
__attribute__((__unused__)).

We amend the debug output in coro.h to avoid similar warnings when print
output is disabled (the default).

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/coro.h: Use a variadic macro for PRINTF to
	avoid unused warnings when output is disabled.
	* g++.dg/coroutines/torture/co-await-04-control-flow.C: Avoid
	unused warnings.
	* g++.dg/coroutines/torture/co-ret-13-template-2.C: Likewise.
	* g++.dg/coroutines/torture/exceptions-test-01-n4849-a.C: Likewise.
	* g++.dg/coroutines/torture/local-var-04-hiding-nested-scopes.C:
	Likewise.
	* g++.dg/coroutines/torture/pr109867.C: Likewise.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

gcc/testsuite/g++.dg/coroutines/coro.h

@@ -135,9 +135,9 @@ namespace coro = std;
 
 #ifndef OUTPUT
 #  define PRINT(X)
-#  define PRINTF (void)
+#  define PRINTF(...)
 #else
 #include <stdio.h>
 #  define PRINT(X) puts(X)
-#  define PRINTF printf
+#  define PRINTF(...) printf(__VA_ARGS__)
 #endif

gcc/testsuite/g++.dg/coroutines/torture/co-await-04-control-flow.C

@@ -1,4 +1,5 @@
 //  { dg-do run }
+//  { dg-additional-options "-Wno-unused-label" }
 
 // Check correct operation of await transform.
 

gcc/testsuite/g++.dg/coroutines/torture/co-ret-13-template-2.C

@@ -13,7 +13,7 @@ coro1
 f (T y) noexcept
 {
   PRINT ("coro1: about to return");
-  T x = y;
+  __attribute__((__unused__)) T x = y;
   co_return 3;
 }
 

gcc/testsuite/g++.dg/coroutines/torture/exceptions-test-01-n4849-a.C

@@ -116,7 +116,6 @@ struct coro1 {
 struct coro1
 n4849_ia_thrower (int k)
 {
-  int caught = 0;
   PRINT ("f: about to return 22");
   co_return 22;
 }

gcc/testsuite/g++.dg/coroutines/torture/local-var-04-hiding-nested-scopes.C

@@ -13,9 +13,9 @@ f (int start) noexcept
 {
   int value = start;
   {
-    int value = start + 5;
+    __attribute__((__unused__)) int value = start + 5;
     {
-	int value = start + 20;
+	__attribute__((__unused__)) int value = start + 20;
     }
     {
 	int value = start + 1;

gcc/testsuite/g++.dg/coroutines/torture/pr109867.C

@@ -16,7 +16,7 @@ struct task
 
 int main()
 {
-    auto t = []() -> task
+    __attribute__((__unused__)) auto t = []() -> task
     {
         co_return 2;
     }();