[llvm][NFC] Suppress -Wunused-result call to write

Commit 87e6f87fe7e343eb656e9b49d30cbb065c086651 adds a call to `::write()`, which may be annotated w/ `warn_unused_result`, leading to `-Wunused-result` failures.

llvm/lib/Support/raw_socket_stream.cpp

@@ -265,7 +265,10 @@ void ListeningSocket::shutdown() {
 
   // Ensure ::poll returns if shutdown is called by a seperate thread
   char Byte = 'A';
-  ::write(PipeFD[1], &Byte, 1);
+  ssize_t written = ::write(PipeFD[1], &Byte, 1);
+
+  // Ignore any write() error
+  (void)written;
 }
 
 ListeningSocket::~ListeningSocket() {