Fix the thread-pool.c compilation

A recent commit removed DIAGNOSTIC_IGNORE_UNUSED_FUNCTION, which was
used in thread-pool.c.  This patch changes this code to use
ATTRIBUTE_UNUSED instead.

Tested by rebuilding.

gdb/ChangeLog
2019-12-12  Tom Tromey  <tromey@adacore.com>

	* gdbsupport/thread-pool.c (set_thread_name): Use
	ATTRIBUTE_UNUSED.

Change-Id: I56d46eaac73690565d0e52db1791411567a918dd

---
 gdb/ChangeLog                |  5 +++++
 gdb/gdbsupport/thread-pool.c | 10 ++--------
 2 files changed, 7 insertions(+), 8 deletions(-)

diff --git a/gdb/gdbsupport/thread-pool.c b/gdb/gdbsupport/thread-pool.c
index 1b3e44c670..fc83ff765f 100644
--- a/gdb/gdbsupport/thread-pool.c
+++ b/gdb/gdbsupport/thread-pool.c
@@ -25,7 +25,6 @@
 #include "gdbsupport/alt-stack.h"
 #include "gdbsupport/block-signals.h"
 #include <algorithm>
-#include "diagnostics.h"
 
 /* On the off chance that we have the pthread library on a Windows
    host, but std::thread is not using it, avoid calling
@@ -40,14 +39,11 @@
 
 #include <pthread.h>
 
-DIAGNOSTIC_PUSH
-DIAGNOSTIC_IGNORE_UNUSED_FUNCTION
-
 /* Handle platform discrepancies in pthread_setname_np: macOS uses a
    single-argument form, while Linux uses a two-argument form.  This
    wrapper handles the difference.  */
 
-static void
+ATTRIBUTE_UNUSED static void
 set_thread_name (int (*set_name) (pthread_t, const char *), const char *name)
 {
   set_name (pthread_self (), name);
@@ -55,14 +51,12 @@ set_thread_name (int (*set_name) (pthread_t, const char *), const char *name)
 
 /* The macOS man page says that pthread_setname_np returns "void", but
    the headers actually declare it returning "int".  */
-static void
+ATTRIBUTE_UNUSED static void
 set_thread_name (int (*set_name) (const char *), const char *name)
 {
   set_name (name);
 }
 
-DIAGNOSTIC_POP
-
 #endif	/* USE_PTHREAD_SETNAME_NP */
 
 namespace gdb