sanitizer: use pragma clang conditionally

[marxin]

Summary

Use the pragma only when clang is defined.

Fixes:
sanitizer_common_libcdep.cpp:101: warning: ignoring ‘#pragma clang diagnostic’ [-Wunknown-pragmas]

compiler-rt/lib/sanitizer_common/sanitizer_common_libcdep.cpp

void MaybeStartBackgroudThread() {
   static bool started = false;
   if (!started) {
     started = true;
     internal_start_thread(BackgroundThread, nullptr);
   }
 }
 
 #  if !SANITIZER_START_BACKGROUND_THREAD_IN_ASAN_INTERNAL
+#    ifdef __clang__
 #    pragma clang diagnostic push
 // We avoid global-constructors to be sure that globals are ready when
 // sanitizers need them. This can happend before global constructors executed.
 // Here we don't mind if thread is started on later stages.
 #    pragma clang diagnostic ignored "-Wglobal-constructors"
+#    endif
 static struct BackgroudThreadStarted {
   BackgroudThreadStarted() { MaybeStartBackgroudThread(); }
 } background_thread_strarter UNUSED;
+#    ifdef __clang__
 #    pragma clang diagnostic pop
-#  endif
+#    endif
+#  endif
 #else
 void MaybeStartBackgroudThread() {}
 #endif
 
 void WriteToSyslog(const char *msg) {
   InternalScopedString msg_copy;
   msg_copy.append("%s", msg);
   const char *p = msg_copy.data();