[clangd][iwyu] explicitly includes `<atomic>`

Compiling clangd with Clang modules and libc++ revealed that
`support/Threading.h` uses `std::atomic` but wasn't including the
correct header.

Differential Revision: https://reviews.llvm.org/D105400

clang-tools-extra/clangd/support/Threading.h

@@ -12,6 +12,7 @@
 #include "support/Context.h"
 #include "llvm/ADT/FunctionExtras.h"
 #include "llvm/ADT/Twine.h"
+#include <atomic>
 #include <cassert>
 #include <condition_variable>
 #include <future>